REESE  LIBRARY 


. 


UNIVERSITY  OF  CALIFORNIA. 

Deceived  7^f. 

Accession  No.      {)  / 0  •   Cla&s  No. 


SPECTRA   OF  DIFFERENT  TYPES. 

(i)  Continuous  spectrum.  (2)  Solar  spectrum.  (3)  Of  Sirius.  (4)  Of  Aldebaran.  (5)  Of  Alpha 
in  Hercules.  (6)  Of  a  red  star.  (7)  Of  a  temporary  star.  (8)  Of  Comet  of  1874.  (9)  °f  the 
nebula  of  Orion.  (10)  Of  sodium.  (11)  Of  hydrogen.  (la)  Of  nitrogen. 


MR.   HUMPHREY   B.   CHAMBERLIN, 

TO  WHOSE  MUNIFICENCE 
THE    AUTHOR   IS   DEEPLY   INDEBTED, 


IS  GRATEFULLY   DEDICATED. 


INTRODUCTION. 


THIS  book  is  intended  for  the  use  of  students  who  have  a  fair 
knowledge  of  elementary  algebra  and  plane  geometry,  and  is 
the  outcome  of  several  years  of  experience  with  classes  of  this  sort. 
The  author  hopes  that  the  volume  will  also  be  acceptable  to  more 
advanced  scholars. 

The  teacher  may  here  be  reminded  of  the  fact  that  among  the 
most  urgent  needs  in  the  study  of  astronomy  is  the  exercise  of  the 
geometric  imagination ;  that  is,  the  faculty  which  forms  mental 
pictures  of  the  relative  positions  of  planes  and  circles,  and  of  the 
motions,  real  and  apparent,  of  the  celestial  bodies.  The  initial 
step  in  astronomical  instruction  is  to  teach  pupils  the  use  of  their 
eyes,  —  to  insist  that  they  observe  the  heavens  and  watch  the  celestial 
motions.  Though  they  may  be  bewildered  by  such  work  at  first, 
they  will  soon  learn  to  delight  in  it,  and  will  derive  much  profit  from 
it.  The  earliest  work  in  the  line  of  observation  is  the  study  of  the 
constellations,  Acquaintance  with  the  principal  stars  of  the  chief 
constellations  visible  in  his  latitude  will  prove  a  source  of  lifelong 
enjoyment  to  the  pupil.  Each  student  should  keep  a  small  blank 
book  in  which  to  make  sketches,  and  to  record  the  results  of  obser- 
vations. 

The  Star  Maps,  at  the  end  of  the  volume,  are  on  a  generous  scale, 
and  include  all  stars  not  fainter  than  the  fifth  magnitude,  from  the 
north  pole  to  40°  of  south  declination.  By  the  use  of  these  maps 
and  star  groups  drawn  on  the  blackboard,  the  teacher  may  greatly 
aid  his  pupils,  who  should  copy  the  pictures  and  then  find  the  corre- 
sponding objects  in  the  sky. 

The  use  of  a  telescope  adds  much  of  interest  to  this  study,  espe- 
cially if  the  students  are  taught  to  manipulate  it  themselves,  and  to 
iv 


INTRODUCTION.  V 

find  by  its  aid  the  telescopic  objects  mentioned  in  Chapter  XIV. 
Even  a  good  opera-glass  is  very  serviceable.  The  observation  exer- 
cises given  at  times  will  be  found  very  helpful,  and  will  assist  in  the 
cultivation  of  the  geometric  imagination.  A  globe,  with  blackboard 
surface,  may  be  useful  in  various  ways,  but  as  soon  as  the  pupils 
have  derived  a  geometric  conception  by  its  aid  they  should  be  led 
at  once  to  transfer  the  mental  picture  to  the  heavens. 

It  is  also  recommended  that  constant  recourse  be  had  to  such 
periodicals  as  "  Popular  Astronomy,"  "  Knowledge,"  etc.,  in  order  to 
follow  the  progress  of  astronomical  research,  month  by  month,  and 
thus  to  supplement  the  text-book.  The  list  of  works,  given  in 
Appendix  VII.,  is  intended  as  a  guide  in  the  selection  of  an  astro- 
nomical library. 

The  optical  principles  of  the  telescope  and  spectroscope  have 
been  explained  very  simply,  for  students  not  familiar  with  descrip- 
tions of  them  in  elementary  text-books  on  physics.  Especial 
attention  has  been  paid  to  the  Meridian  Circle  and  to  the  Equatorial, 
because  accurate  knowledge  of  the  positions  and  motions  of  the 
heavenly  bodies  depends  chiefly  on  observations  made  with  these 
instruments. 

The  purely  descriptive  matter  about  the  sun,  moon,  planets,  etc., 
has  been  kept  quite  free  from  such  statistics  as  the  values  of  the 
masses  of  the  planets,  and  the  intensity  of  the  pull  of  gravitation  at 
the  surface  of  each.  The  student  should,  however,  learn  the  distance, 
diameter,  time  of  revolution,  and  time  of  rotation  of  each  planet. 
More  extended  data  for  purposes  of  reference  are  to  be  found  in  the 
Appendices. 

In  this  edition  the  results  of  the  latest  important  investigations 
and  discoveries  have  been  stated.  The  work  of  the  Lick  Observa- 
tory, as  set  forth  in  the  publications  of  the  Astronomical  Society  of 
the  Pacific,  merits  and  has  received  much  attention.  The  columns 
of  astronomical  periodicals  have  furnished  a  large  amount  of  reliable 
information.1  The  author  will  welcome  for  a  second  edition  any 
suggestions  or  corrections. 

The  Exercises,  which  are  a  special  feature  of  this  book,  and  are 
placed  at  the  end  of  each  chapter,  will  be  of  great  help  to  the  pupils 

1  A  good  Star  Atlas  is  a  desideratum.  Almost  every  school-book  publishing  house 
can  furnish  one.  It  will  supplement  the  Star  Maps  at  the  end  of  this  volume. 


VI  INTRODUCTION. 

in  reviewing  the  lessons,  and  also  to  the  teacher  in  the  work  of  the 
class-room. 

The  Appendices  contain,  along  with  other  useful  material,  questions 
for  examination,  topics  for  essays,  and  short  reviews  of  a  number  of 
valuable  works  on  Astronomy  suitable  for  reference  and  general 
reading. 

A  large  number  of  the  illustrations  have  never  before  appeared 
in  any  text-book.  For  many  of  the  finer  ones  the  author  is  in- 
debted to  Prof.  E.  S.  Holden,  Prof.  Wm.  W.  Payne,  Mr.  A.  Cowper 
Ranyard,  and  Dr.  E.  E.  Barnard.  Prof.  E.  C.  Pickering  kindly 
furnished  a  fine  set  of  lithographs,  made  from  observations  at  Har- 
vard College  Observatory,  many  of  which  have  been  reproduced. 
Prof.  G.  E.  Hale  has  contributed  photographs  of  the  solar  disturb- 
ance of  July  15,  1892.  Messrs.  G.  W.  Saegmuller,  of  Washington, 
D.  C.,  and  J.  A.  Brashear,  of  Allegheny,  Pa.,  supplied  the  pictures 
of  some  of  the  instruments.  The  author  desires  to  state  that  he  is 
specially  indebted  to  his  wife,  Fannie  Shattuck  Howe,  for  her  assist- 
ance in  the  preparation  of  the  manuscript. 

UNIVERSITY  PARK,  COLORADO, 
1896. 


CONTENTS. 


CHAPTER   I. 

GENERAL  SURVEY  OF  THE  HEAVENS. 

PAGE 

Celestial  Objects  classified ;  the  Star  Maps  explained  ;  Names  of  the 
Constellations ;  how  to  find  the  Constellations ;  Hints  on  Constella- 
tion Study i 


CHAPTER   II. 

APPARENT   DAILY   MOTION   OF   THE   STARS. 

The   Daily   Motion ;    the   Celestial    Sphere ;    the   Celestial   Equator  and 

Horizon;  Exercises        6 

CHAPTER   III. 

THE   TELESCOPE. 

Reflection  and  Refraction  of  Light;  Lenses;  Formation  of  an  Image; 
Object-glass  and  Eyepieces ;  Dispersion  of  Light ;  Achromatism ; 
Refractors  and  Reflectors  ;  Equatorial  Mountings  ;  Exercises  ....  19 

CHAPTER   IV. 

THE   SUN. 

Its  Distance  and  Diameter ;  how  to  Observe  it  with  a  Small  Telescope ; 
Photosphere;  Faculae;  Spots;  Solar  Disturbances;  Magnetic  Storms; 
the  Spectroscope ;  Laws  of  Spectrum  Analysis ;  the  Chromosphere ; 
Prominences  ;  the  Corona  ;  Light  and  Heat;  Constitution;  Exercises  .  36 

vii 


Vlll  CONTENTS. 

CHAPTER   V. 

THE   EARTH. 

PAGE 

Dimensions;  Latitude  and  Longitude;  its  Orbit;  the  Ecliptic;  the  Equi- 
nox and  Solstices ;  the  Zodiac ;  the  Day ;  the  Seasons ;  Precession 
and  the  Calendar;  Aberration;  Atmospheric  Refraction;  Twilight; 
Exercises 64 

CHAPTER  VI. 

CELESTIAL   MEASUREMENTS. 

Circles  of  Reference  ;  Parallax;  Time;  Solar  and  Sidereal  Days  ;  Civil  and 
Astronomical  Days;  Mean  Solar  and  Sidereal  Time;  Standard  Time, 
and  its  Determination  by  Means  of  a  Meridian  Circle  and  a  Chrono- 
graph ;  Determination  of  Latitude  and  Longitude ;  Exercises  ....  87 

CHAPTER   VII. 

THE   MOON   AND    ECLIPSES. 

Distance;  Diameter;  Orbit;  Period;  Rotation;  Librations  ;  Phases;  Plains; 
Craters ;  Mountains ;  Water  and  Air  ;  Light  and  Heat ;  Effect  on  the 
Weather;  Eclipses,  Solar  and  Lunar ;  Exercises 108 

CHAPTER  VIII. 

MOTIONS   OF   THE   PLANETS. 

Their  Orbits;  Newton's  and  Kepler's  Laws ;  Aspects;  Periods;  Exercises.     133 

CHAPTER   IX. 

MERCURY,  VENUS,  MARS,  THE    ASTEROIDS. 

Their   Distance;    Diameter;    Revolution;    Rotation;    Phases;    Satellites; 

Atmosphere  ;  Telescopic  Appearance  ;  Physical  Condition  ;    Exercises.     143 

CHAPTER  X. 

JUPITER,  SATURN,  URANUS,  NEPTUNE. 

Their  Distance  ;  Diameter  ;  Revolution  ;  Rotation ;  Discovery ;  Satellites  ; 

Atmosphere  ;  Telescopic  Appearance  ;   Physical  Condition  ;    Exercises.     164 


CONTENTS.  IX 

CHAPTER   XI. 

COMETS   AND    METEORS. 

PAGE 

Comets,  their  Discovery  ;  Designation ;  Parts  ;  Orbits ;  Appearances;  Tails; 
Mass ;  Light ;  Spectra;  Fate.  —  Meteors,  their  Classes;  Paths;  Light 
and  Heat;  Constituents;  Showers;  Orbits,  —  Exercises 186 

CHAPTER   XII. 

THE   FIXED   STARS. 

Number;  Milky  Way;  Constellations;  Names;  Magnitudes;  Dimen- 
sions; Distances;  Clusters;  Parallax;  Spectra;  Motions;  Double  and 
Multiple  ;  Variable ;  Exercises 225 

CHAPTER  XIII. 

THE   NEBUUE. 

Various   Forms ;  Spectra ;  Notable   Ones  ;  the   Nebular  Hypothesis ;  the 

Future  of  the  Visible  Universe 257 

CHAPTER  XIV. 

THE   CONSTELLATIONS    IN   DETAIL. 

The  Greek  Alphabet ;  Detailed  Descriptions  of  the  Constellations  visible 
in  the  United  States,  with  Tabular  Lists  of  Prominent  Double  Stars, 
Clusters,  Nebulae,  Colored  and  Variable  Stars 272 


APPENDICES. 

I.    Names  of  Stars .     .     ,     .  303 

II.    Astronomical  Constants 304 

III.  Planetary  Data t 305 

IV.  Landmarks  in  the  History  of  Astronomy .     .     .  307 

V.    Topics  for  Essays „  313 

VI.    Queries  for  Use  in  Reviews  and  Examinations 315 

VII.   List  of  Reference  Books 320 


INDEX „    327 

STAR  MAPS ..,.,'».    342 


LIST   OF   ILLUSTRATIONS. 


PAGE 

FRONTISPIECE.    Spectra  of  Different  Types 

Fig.     i.     Solar  Prominences     .    .   opposite  i 

"       2.     Revolution  of  the  Sphere        .     .  7 

"       3.     The  Great  Dipper  and  Polaris     .  8 

Figs.  4-6.  Diagrams  illustrating  Definitions  9 
"      7, 8.  Apparent    Daily    Motion   of   the 

Stars        16 

Fig.     9.     Apparent    Daily     Motion  of  the 

Stars        17 

"      10.     Reflection  by  a  Plane  Mirror  .     .  19 

"      ii.     Reflection  by  a  Concave  Mirror    .  20 

"      12.     Refraction 20 

"      13.     Refraction  by  Prisms     ....  20 

"      14.     Lenses 21 

"     15.     Rays  made  Divergent     ....  21 

"      16.     Visual  Angle 21 

"      17.     Object  and  Image 22 

18.  Lens  as  Eyepiece 23 

19.  Object-Glass  and  Eyepiece       .     .  23 

20.  Dispersion 25 

21.  Dispersion  Corrected     ....  25 

22.  Achromatic  Object-Glass     ...  26 

23.  Portrait  of  Alvan  Clark       ...  26 

24.  Eyepieces 27 

25.  Path  of  Rays  of   Light  in  a  Re- 

flector       27 

26.  A  Newtonian  Reflector  made  by 

Brashear 28 

27.  Lord  Rosse's  Six-foot  Reflector    .  29 

28.  Scheme  of  an  Equatorial  Mount- 

ing        30 

29.  A  German  Equatorial    .     .     .     .  31 

30.  The  Lick  Telescope       ....  32 

31.  The  Sun's  Image  on  a  Screen       .  37 

32.  Absorption  of  Light  by  the  Sun's 

Atmosphere 37 

33.  Faculae  observed  Visually   ...  38 

34.  Sun  Spot 40 

35.  Large  Sun  Spot 41 

36.  The  Sun  at  the  Time  of  a  Spot 

Maximum 42 

37.  Photographs   of   the  Disturbance 

of  July  15,  1892 43 


PAGE 

Fig.  38.     Cyclonic  Motion        44 

"     39.     A    Spectroscope  made    by    Bra- 
shear       47 

"  40.  Plan  of  a  Spectroscope  ....  47 
"  41.  Slit  of  a  Spectroscope  ....  48 
"  42.  Production  of  Spectra  ....  49 

"      43.     Spectra 49 

"      44.     Protrait  of  Kirchhoff       .     .     .     .     50 

"     45.     A  Geissler's  Tube 50 

"  46.  A  Portion  of  the  Solar  Spectrum  51 
"  47.  Correspondence  of  Bright  and 

Dark  Lines  in  Two  Spectra  .  51 
"  48.  Solar  Prominences  .  .  opposite  53 
"  49.  A  Spectroscope  attached  to  a 

Telescope 54 

"      50.     The  Corona  on  July  29,  1878  .     .     55 
"      51.     The    Corona,    photographed     on 
December  21,  1889     .... 
"      52.     Illustrations  of  Schaeberle's  The- 
ory of  the  Corona       .... 
"      53<z.     A  Drawing  of  the  Corona       .     . 
Figs.  53^,  53^:.     Drawings  of  the  Corona     . 
Fig-  53^-     A  Photograph  of  the  Inner  Co- 
rona     58 

"     54.     Electrical  Appearances  similar  to 

the  Corona 59 

"     55.     Production  of  Heat  by  the  Action 

of  Gravity 61 

"      56.     Direction  of  the  Plumb-line     .     .     65 
"      57.     How  to  find  the  Earth's  Diame- 
ter       

"      58.     Latitude  and  Longitude 

59.  The  Astronomical  Latitude 

60.  An  Ellipse 68 

61.  Illustration  of  the  Ecliptic        .     .     69 

62.  The  Obliquity  of  the  Ecliptic       .     70 

63.  The  Ecliptic  and  the  Equator       .     70 

64.  The  Sun's  Daily  Motion  among 

the  Stars .     71 

65.  A  Spinning  Top 71 

66.  Successive  Positions  of  the  Earth's 

Equator 72 

67.  An  Orange  Half  Submerged    .     .     72 


56 

57 

57 
53 


56 

66 

67 


LIST    OF    ILLUSTRATIONS. 


XI 


PAGE 
Fig.  68.  Locating  the  North  Pole  .  .  • .  74 

"  69.  The  Midnight  Sun 74 

"  70.  Effect  of  the  Slant  of  the  Sun's 

Rays 75 

"  71.  The  Earth's  Equatorial  Ring  .  75 

"  72.  A  Leaning  Top 76 

"  73.  Positions  of  the  Axis  of  the  Top  76 
"  74.  The  Precessional  Motion  of  the 

Earth's  Equator 77 

"  75.  Path  of  North  Celestial  Pole 

among  the  Stars 78 

"  76.  Different  Kinds  of  Years  ...  79 
"  77.  Illustration  of  Aberration  .  81 

"  78.  Refraction 82 

"  79.  Gravity  and  the  Earth's  Rotation  82 
Figs.  80-83.  Illumination  of  the  Earth  by 

the  Sun 85 

Fig.  84.  Circles  of  Reference  ....  87 
"  85.  Equator,  Horizon,  Meridian, 

etc 89 

"•  86.  Parallax 90 

"  87.  Unequal  Lengths  of  Apparent 

Solar  Days      ......       91 

"  88.  Variable  Motion  in  Hour  Angle  92 

"  89.  A  Standard  Clock 95 

"  90.  A  Portable  Meridian  Circle  .  .  96 

"  91.  A  Reticle 97 

"  92.  Motion  of  a  Star's  Image  .  .  97 

"  93.  A  Chronograph 99 

"  94.  A  Chronographic  Record  .  .  100 
"  95.  Determination  of  Latitude  .  .  100 
"  96.  Latitude  found  by  Observation 

of  Polaris 101 

"  97.  An  Engineer's  Transit  .  .  .  102 

"  98.  A  Chronometer 103 

"  99.  A  Sextant 104 

"  100.  Orbits  of  the  Earth  and  Moon  .  108 
"  101.  Sidereal  and  Synodic  Periods  .  109 
"  102.  Illustration  of  the  Moon's  Rotation  109 

Figs.  103,  104.  Libration no 

Fig.  105.  The  Moon  Illuminated  .  .  .  IIT 
"  106.  The  Moon's  Phases  .  .  .  .  in 
"  107.  The  Moon  :  Photographed  at 

Lick  Observatory  .  .  .  .  112 
(t  108.  Skeleton  Map  of  the  Moon  .  .  114 
"  109.  Conspicuous  Craters  of  the 

Moon  .  . 115 

"  TIO.  The  Crater  Copernicus  .  .  .  116 
;'  in.  Portrait  of  Copernicus  .  .  .  117 
"  112.  The  Terrestrial  Crater  Vesuvius  118 
"  113.  The  Lunar  Apennines  .  .  .  119 
"  114.  The  Crater  Vendelinus  :  From 

Photograph '120 

"  115.  Umbra  and  Penumbra  of  the 

Earth's  Shadow 125 

"  116.  Umbra  and  Penumbra  of  the 

Moon's  Shadow 125 


Fig.  117. 
"     118. 

"     119. 

"        120. 

"        121. 
«'        122. 


I24. 
125. 

126. 


128. 
129. 
'3°. 
'3'. 
132. 

'33- 
134- 
'35- 
136- 
137. 

138. 
i39. 
140. 

141. 
142. 

'i43- 
144- 
i45- 
146. 

i47- 
148. 
149. 

150. 
151. 
152. 

I53- 
154- 
155- 
156. 

157. 
158. 
159- 

160. 
161. 


PAGE 

Cross-section  of  a  Shadow     .     .  125 
Beginning   of   a   Total  Lunar 

Eclipse 126 

Lunar   Eclipse,   Jan.  1888  opp.  126 
Path  of  Central  Line  of  Eclipse, 

May  27,  1900 127 

Cause  of  Annular  Eclipse     .     .  128 
Appearance  of  Sun  during  an 

Annular  Eclipse  ....  128 
Portrait  of  Sir  Isaac  Newton  .  134 
Portrait  of  Kepler  ....  135 
Equal  Areas  in  Equal  Times  .  136 
Aspects  of  the  Planets  .  .  .  137 
Apparent  M  ovement  of  a  Supe- 
rior Planet 139 

Relative  Sizes  of  the  Planets    .  143 

A  Transit  of  Venus    ....  147 

The  Black  Drop 147 

Portrait  of  Galileo       ....  148 

The  Ring  of  Light      ....  149 

Mars:  Drawn  by  Barnard    .     .  151 

The  Canals  of  Mars    ....  155 

The  Zone  of  Asteroids     .     .     .  160 

Telescopic  Experiment   .     .     .  163 
Jupiter,  as  seen  with  the  Lick 

Telescope 166 

Orbits  of  the  Major  Satellites  .  168 

Phenomena  of  the  Satellites      .  169 
Markings  seen  with  the  Lick 

Telescope 169 

Jupiter    and    the    Orbit  of    the 

Fifth  Satellite 170 

Saturn,  as  seen  with  the  Lick 

Telescope 172 

Different  Positions  of  the  Rings  1 74 

Old  Drawing  of  Saturn    .     .     .  175 

Portrait  of  Sir  William  Herschel  177 

Portrait  of  John  Couch  Adams  180 

Conic  Sections 189 

Varieties  of  Orbits       ....  189 
Orbits  of  some  Comets  of  Jupi- 
ter's Family 191 

A  Jet 192 

Companions  of  Brooks's  Comet  193 

Development  of  a  Tail     .     .     .  194 

Comet's  Tail.     Type  I.  .     .     .  195 

Comet's  Tail.     Type  II.      .     .  195 

Comet's  Tail.     Type  III.    .     .  195 

Comet  of  1528 198 

Comet  of  i86r 200 

The  Great  Comet  of  1882     .     .  203 
Nucleus  of  the  Great  Comet  of 

1882 204 

Swift's  Comet:     Photographed 

by  Barnard 205 

Brooks's  Comet :  Photographed 

by  Barnard      ......  207 


Xll 


LIST    OF    ILLUSTRATIONS. 


PAGE 

Fig. 

162. 

Meteor    seen  at    Bassein,   Bur- 

Fig.  179. 

2IO 

'"    180. 

n 

rfis 

212 

"     181. 

«i 

10j. 

164. 

The  Canyon  Diablo  Meteorite  . 

213 

"     182! 

41 

I65. 

Relative  Frequency  of  Meteors 

"     183. 

in  the  Morning  and  Evening  . 

215 

"     184. 

•it 

TfiA 

216 

"      i8t: 

44 

IOO* 

167. 

The  Orbit  of  the  August  Shower 

218 

105. 

"     186. 

it 

168. 

Capture  of  the  Leonids    .     .     . 

220 

"     187. 

•« 

169. 

Stars  Visible  to  the  Naked  Eye 

224 

4i 

170. 

The   Yerkes  Telescope   at  the 

"     188. 

World's  Fair,  1893    .     •     •     • 

226 

«     189- 

4t 

171. 

A  Portion  of  the  Milky  Way    . 

227 

4t 

172. 

Plant-like  Structure    .... 

228 

"     190. 

u 

173. 

The  Great  Cluster  in  Hercules 

232 

U 

174. 

The   Cluster  Omega  Centauri  : 

"     191. 

Photographed  by  Dr.   Gill  at 

the  Cape  of  Good  Hope    .     . 

233 

»    192. 

(I 

T  7S 

Stellar  Parallax                 .     .     . 

2^4. 

(l 

xo* 

176. 

Method    of    observing    Stellar 

•*J4- 

Parallax 

21A. 

"      10^ 

tt 

177. 

Relation  of  Parallax  to  Distance 

*JT 

235 

*%>• 

«    194- 

ii 

17,8. 

Proper  Motions  of  the  Pleiades 

240 

"    195- 

PAGE 

Double  Stars 242 

A  Spectroscopic  Binary   .     .     .  244 

Multiple  Stars   ......  245 

Portrait  of  Tycho  Brahe  .  .  247 

Tycho's  Star  in  Cassiopeia  .  248 

How  to  find  Algol  ....  249 

Y  Cygni 250 

Real  Velocity  of  a  Star  .  .  .  254 
The  Pleiades:  Photographed 

by  Roberts 258 

The  Trifid  Nebula  ....  260 
The  Nebula  in  Andromeda : 

Photographed  by  Roberts  .  26 1 
The  Nebula  in  Orion  :  Drawn 

by  Bond 262 

The  Ring  Nebula  in  Lyra : 

Drawn  by  Bond 264 

The  Spiral  Nebula  in  Canes 

Venatici :     Photographed    by 

Roberts 265 

Portrait  of  La  Place  ....  266 

The  Star  Finder 295 

The  Declination  Circle  .  .  .  295 


THE    SUN. 


SOLAR    PROMINENCES. 


nr  c      /  '         OF  THE 

1  o    (UNIVERSITY  ) 


DESCRIPTIVE    ASTRONOMY. 


CHAPTER  I. 

GENERAL  SURVEY  OF  THE  HEAVENS. 

"  The  sky 

Spreads  like  an  ocean  hung  on  high, 
Bespangled  with  those  isles  of  light 
So  wildly,  spiritually  bright. 
Who  ever  gazed  upon  them  shining, 
And  turned  to  earth  without  repining, 
Nor  wished  for  wings  to  flee  away, 
And  mix  with  their  eternal  ray  ? " 

BYRON. 

•1.  The  Fixed  Stars. — -The  fixed  stars  are  points  of  light,  of  vari- 
ous degrees  of  brightness,  which  bestrew  the  sky.  They  are  called 
fixed,  because,  as  seen  with  the  naked  eye,  they  do  not  change  their 
relative  positions  from  year  to  year. 

In  the  earliest  ages  men  divided  them  into  various  groups,  which 
we  call  constellations ;  the  appearance  of  each  of  these  constella- 
tions is  almost  the  same  to-day  as  when  it  was  named  by  the 
ancients. 

A  star  just  visible  to  an  average  eye  is  said  to  be  of  the  sixth 
magnitude ;  one  a  little  brighter  is  said  to  be  of  the  fifth  magnitude, 
and  so  on,  a  few  of  the  brightest  being  called  first  magnitude  stars. 
All  fixed  stars  are  at  inconceivably  great  distances  from  us. 

2.  The  Planets. — The  word  planet  is  derived  from  a  Greek  word 
meaning  "a  wanderer."  The  designation  is  applied  to  certain  star- 
like  objects  which  appear  to  move  among  the  fixed  stars.  The 


2  DESCRIPTIVE    ASTRONOMY. 

brightest  ones  have  received  the  names  of  ancient  divinities,  as 
Jupiter,  Saturn,  and  Venus.  They  revolve  about  the  sun  in  paths 
nearly  circular ;  the  earth  is  considered  one  of  them. 

3.  The  Moon.  —  This  familiar  object  revolves  about  the  earth  in 
27 J  days.     It  is  the  nearest  of  the  celestial  bodies,  being  a  little 
less  than  a  quarter  of  a  million  of  miles  away.     It  belongs  to  the 
class  of  objects  known  as  satellites,  which  revolve  about  the  planets, 
held  fast  by  their  attractive  force. 

4.  The  Sun.  —  The  sun,   like  the  moon,   is  so    familiar  that  no 
particular  description  of  it  is    needed    here.     Though  its  distance 
from  the  earth  is  nearly  93,000000  miles,  it  is  very  much  nearer  to 
us  than  any  one   of  the  fixed   stars.     To  its  abounding  light  and 
heat  we  owe  the  preservation  of  our  lives,  the  maintenance  of  our 
vigor,  the  physical  comforts  which  we  enjoy,  and   the  marvellous 
beauties  of  nature. 

5.  Comets. — The  word  "  comet "  is  derived  from  a  Greek  word 
meaning  "  long-haired."     Comets  are  usually  invisible  to  the  naked 
eye,  but  sometimes  attain  great  splendor  and  beauty.     In  ancient 
and  mediaeval  times  their  appearance  was  usually  regarded  as  a  dire 
omen.     Even  in  1861  it  was  rumored  in  Italy  that  the  great  comet 
of  that  year  presaged  the  death  of  Pope  Pius  IX. 

6.  Meteors.  —  These   are'  evanescent  objects  which  flash  across 
the  sky,  and  usually  fade  from  sight  in  a  few  seconds.     Occasionally 
they   are  so   brilliant   as   to    be    seen    in   broad  daylight,   and    are 
accompanied  by  terrific  detonations. 

7.  Nebulae.  —  Nebulae,  as  their  name   implies,   are   cloudlike    in 
appearance.     A  few  of  them  are  conspicuous  enough  to  be  faintly 
seen  without  telescopic  aid,  and  appear  as  feeble  patches  of  light 
on  the  dark  background  of  the   sky.     They  are  large  and  diffuse 
masses  of  matter,  at  vast  distances  from  us. 

8.  The  Star  Maps.  —  In  the  maps  at  the  end    of  this  book,  the 
magnitudes  are  indicated  very  simply ;   especial  care  has  been  taken 
in  drawing  the  dotted  lines  connecting  the  stars  in  the  constellations, 
so  that  figures  easily  remembered  may  be  obtained.     Careful  direc- 
tions are  given  for  learning  the  constellations,  together  with  lists  of 
telescopic  objects,  most  of  which  are  within  the  power  of  a  three- 
inch  telescope.     Stars  of  the  first  magnitude  are  indicated  by  heavy 
black  dots.     Those  of  the  fifth  magnitude  are  represented  by  small 


GENERAL  SURVEY  OF  THE  HEAVENS. 


dots.  A  star  of  the  second  magnitude  has  two  short  arms  project- 
ing from  a  central  dot.  Stars  of  the  third  and  fourth  magnitude 
have,  respectively,  three  and  four  projecting  arms.  The  figures  at 
the  top  and  bottom  of  each  map,  except  the  first,  denote  the  right 
ascensions  of  the  stars ;  those  at  the  sides  indicate  the  declinations. 
On  Map  I.  right  ascensions  are  given  around  the  circumference, 
declinations  along  a  diameter.  These  terms  are  explained  in  §  122. 
They  are  not  needed  in  learning  the  constellations.  Each  con- 
stellation is  bounded  by  a  heavy  dotted  line,  and  its  name  is  printed 
in  large  letters.  The  proper  names  of  the  brightest  stars,  such  as 
Sirius,  Vega,  etc.,  are  given.  Most  of  the  stars  are  marked  by 
letters  or  numbers.  The  name  of  such  a  star  is  formed  by  adding 
to  its  letter  the  Latin  genitive  of  the  name  of  the  constellation  in 
which  it  lies.  Thus  the  star  m  in  Orion  is  called  m  Orionis. 

9.  Names  of  the  Constellations.  —  The  names  of  the  constellations 
shown  on  the  Star  Maps  are  given  in  the  table  below.  The  Greek 
alphabet  is  found  in  §  405. 


LATIN  (Nominative) 

An-drom'-e-da 

A-qua'-ri-us 

Aquila  (Ak'-wi-la) 

Ar-go  Na-vis 

Aries  (A'-ri-ez) 

Au-ri'-ga 

Bootes  (Bo-6'-tez) 

Cam-el-o-par'-dus 

Can-cer 

Canes  Venatici  (Ka'-nez  Ve-nat'-i-si) 

Ca-nis  Ma-jor 

Ca-nis  MT-nor 

Cap-ri-cor'-nus 

Cassiopeia  (Kas-si-6-pe'-ya) 

Centaurus  (Sen-taw'-rus) 

Ce'-phe-us 

Ce-tus 

Co-lum'ba 

Coma  Berenices 

Co-ro'-na  Bo-re-a'-lis 

Corvus 

Cra-ter 

Cyg'-nus 

Del-phi'-nus 

Dra-co 


LATIN  (Genitive) 

Andromedae 
Aquarii 
Aquilae 
Argus 
Arietis 
Aurigae 
Bootis 

Camelopardi 
Cancri 
Can.  Ven. 
Canis  Majo'ris 
Canis  Mino'ris 
Capricorni 
Cassiope'iae 
Centau'ri 
Cephei 
Ceti 

Columbae 
Comae  Berenl'ces 
Coronae  Borea'lis 
Corvi 
•  Cra'teris 
Cygni 
Delphihi 
Draco'nis 


ENGLISH 
Andromeda 
The  Water  Carrier 
The  Eagle 
The  Ship 
The  Ram 
The  Charioteer 
The  Bear  Keeper 
The  Camelopard 
The  Crab 
The  Hunting  Dogs 
The  Great  Dog 
The  Little  Dog 
The  Goat 

The  Lady  in  the  Chair 
The  Centaur 
Cepheus 
The  Whale 
The  Dove 

The  Hair  of  Berenice 
The  Northern  Crown 
The  Crow 
The  Cup 
The  Swan 
The  Dolphin 
The  Dragon 


DESCRIPTIVE    ASTRONOMY. 


LATIN  (Nominative) 

Equuleus  (E-kwu'-le-us) 

E-rid'-a-nus 

Gem'-i-m 

Her'-cu-les 

Hy'-dra 

La-cer'-ta 

Le-o 

Le-o  Mi-nor 

Le-pus 

Li-bra 

Lu-pus 

Lynx 

Ly'-ra 

Mohoceros  (Mo-nos'-e-ros) 

Oph-i-u'-chus 

O-rT'-on 

Peg'-a-sus 

Per'-se-us 

Pisces  (Pis'-sez) 

Pis'-cis  Aus-tra'-lis 

Sagitta  (Sa-jit'-ta) 

Sagittarius  (Saj-i-ta'-ri-us) 

Scor'-pi-6 

Sculp-tor 

Scu-tum 

Serpens  (Ser'-penz) 

Sextans  (Seks'-tanz) 

Taurus  (Taw'-rus) 

Triangulum  (Tri-ang'-gu-lum) 

Ur'-sa  Ma-jor 

Ur'-sa  Mi-nor 

Vir'-go 

Vul-pec'-u-la 


LATIN  (Genitive) 
Equu'lei 
Eridani 
Gemino'rum 
Herculis 
Hydrae 
Lacertae 
Leo'nis 

Leo'nis  Mino'ris 
Lep'-o-ris 
Libras 
Lupi 
Lyncis 
Lyras 

Monocero'tis 
Ophiuchi 
Orionis 
Pegasi 
Persei 
Piscium 
Piscis  Austr. 
Sagittae 
Sagittarii 
Scorpii 
Sculpto'ris 
Scuti 
Serpentis 
Sextantis 
Tauri 
Trianguli 
Ursae  Majo'ris 
Ursae  Mino'ris 
Vir'-gTnis 
Vulpeculae 


ENGLISH 

The  Little  Horse 

The  River 

The  Twins 

Hercules 

The  Snake 

The  Lizard 

The  Lion 

The  Little  Lion 

The  Hare 

The  Scales 

The  Wolf 

The  Lynx 

The  Harp 

The  Unicorn 

The  Serpent  Bearer 

Orion 

The  Winged  Horse 

Perseus 

The  Fishes 

The  Southern  Fish 

The  Arrow 

The  Archer 

The  Scorpion 

The  Sculptor 

The  Shield 

The  Serpent 

The  Sextant 

The  Bull 

The  Triangle 

The  Great  Bear 

The  Little  Bear 

The  Virgin 

The  Fox 


10.  How  to  Find  the  Northern  Constellations.  —  The  constellations 
visible  in  the  northern  sky  are  to  be  found  on  Map  I.  The  appear- 
ance of  these  constellations  at  8  o'clock  on  any  evening  may  be 
found  by  using  the  dates  around  the  circumference  of  the  map.  If 
the  aspect  of  the  northern  sky  on  April  ist,  for  instance,  is  desired, 
hold  the  map  so  that  the  date  April  ist  shall  be  uppermost.  Face 
the  north  and  hold  the  map  up  toward  the  sky. 

The  Great  Dipper,  which  is  a  portion  of  the  constellation  of  the 
Great  Bear  (Ursa  Major),  can  then  be  found  readily. 

After  the  Great  Dipper  has  been  fixed  in  mind,  the  Pole  Star 
can  be  located  by  the  help  of  Fig.  3.  Cassiopeia  is  on  the  opposite 


GENERAL  SURVEY  OF  THE  HEAVENS.  5 

side  of  the  Pole  Star  from  the  Great  Dipper,  and  at  about  the  same 
distance  from  it.  The  five  brightest  stars  in  this  constellation  form 
a  straggling  W  and  are  quickly  discovered. 

Beginning  at  the  Pole  Star,  one  can  then  trace  out  the  Little  Bear 
(Ursa  Minor)  with  the  assistance  of  the  map. 

When  the  outlines  of  these  constellations  have  been  learned  thor- 
oughly, there  will  be  very  little  difficulty  in  becoming  acquainted 
with  the  adjacent  ones. 

11.  How  to  Find  the  Constellations  in  the  South.  —  These  constella- 
tions are  pictured  on  Maps  II. -V. 

The  stars  underneath  any  particular  date  at  the  top  of  the  map 
are  to  be  seen  in  the  south  at  8  P.  M.  on  that  date.  For  example, 
Map  II.  shows  that  on  February  i6th  Orion  is  in  the  south  at  8  P.  M. 
In  winter,  Orion  is  the  best  southern  constellation  to  learn  first, 
because  of  its  conspicuousness. 

In  spring,  Leo  (see  Map  III.)  is  recommended  as  a  starting 
point;  in  summer,  Scorpio  (see  Map  IV.)  will  answer  the  same 
purpose;  it  will  be  seen  low  down  in  the  south.  For  autumn, 
Pegasus  (see  Map  V.)  is  available.  Four  of  the  principal  stars  of 
this  constellation  form  a  large  square. 

12.  Hints   on   Constellation   Study.  —  When   trying    to    learn    any 
particular  constellation,  the  student  should  find  from  the  maps  about 
half  a  dozen  or  a  dozen  of  its  principal  stars.     The  configuration  of 
these   should   be    impressed  upon  the  mind  so  thoroughly  that  a 
drawing   showing    their    relative    positions   can    be    made    without 
looking  at  the  maps.     With  this  picture  well  in  mind,  the  student 
may  confront  the  sky.     It  is  well  to  note  the  brightness  and  color 
of  any  first  magnitude   star  in   the  constellation,   and  to   learn   its 
name.    In  many  cases  one  can  trace  in  a  constellation  a  resemblance 
to  the  object  after  which  it  is  named.    Orion,  for  instance,  resembles 
the  figure  of  a  man.     The  best  way  to  secure  a  thorough  acquaint- 
ance with  the  constellations  which    have  been  learned  is  to    draw 
them  from  memory  frequently,  and  to  look  at  the  heavens  whenever 
occasion  offers. 

The  descriptive  matter  in  Chapter  XIV.  will  be  found  useful  in 
this  work. 


DESCRIPTIVE    ASTRONOMY. 


CHAPTER    II. 

APPARENT   DAILY   MOTION   OF   THE   STARS. 

•  "  The  sad  and  solemn  night 

Hath  yet  her  multitude  of  cheerful  fires  ; 

The  glorious  host  of  light 

Walk  the  dark  hemisphere  till  she  retires ; 

All  through  her  silent  watches,  gliding  slow, 

Her  constellations  come,  and  climb  the  heavens,  and  go." 

BRYANT. 

13.  The  Daily  Motion.  —  The  most  casual  observer  cannot  fail  to 
notice  that  the  majority  of  the  visible  stars  daily  rise,  travel  across 
the  sky,  and  set. 

The  Greek  philosopher,  Pythagoras,  is  said  to  have  taught  that 
the  thousands  of  fixed  stars  which  stud  the  sky  were  set  in  a  crystal 
sphere,  which,  by  its  daily  revolution,  carried  them  around  the  earth 
as  a  centre. 

By  this  theory  the  apparent  daily  motion  of  the  stars  was  ex- 
plained very  simply  and  accurately,  so  far  as  the  unassisted  eye 
could  judge. 

14.  Cause  of  the  Motion. — When  a  passenger   train,  which    has 
been  standing  near  a  motionless  freight  train,  starts  without   per- 
ceptible jar,  a  passenger  looking  at  the  freight  train  has  the  impres- 
sion at  first  that  the  latter  train  is  moving,  and  his  own  standing 
still.     A  person  on  the  deck  of  a  steamer,  which  is  slowly  turning 
about  in  a  harbor,  sees  the  objects  on  shore  apparently  revolving 
about  him.    If  a  visitor  to  an  astronomical  observatory  looks  upward 
at  the  dome  while  it  is  being  turned,  the  floor  on  which  he  stands 
seems  to  be  revolving.     Similarly,  the  apparent  daily  revolution  of 
the  heavens  is  an  illusion. 

The  earth  turns  upon  its  axis  once  a  day,  but  makes  the  rotation 
without  noise  or  jar,  so  that  the  observer  is  unconscious  of  it,  and  is 
led  to  think  that  the  earth  is  at  rest,  and  that  the  sky  moves. 

15.  The  Celestial  Sphere.  —  As  one  looks  by  night  at  the  heavenly 
bodies,  they  all  seem  to  lie  on  the  surface  of  an  immense  dome. 


APPARENT    DAILY    MOTION    OF   THE    STARS.  7 

Were  the  earth  to  vanish  suddenly,  the  observer  would  seem  to  be 
in  the  centre  of  a  hollow  star-spangled  sphere. 

The  distance  from  his  eye  to  any  celestial  object  he  could  not 
tell.  But  his  reason  would  quickly  declare  that  the  stars  might 
really  be  at  widely  different  distances  from  him,  while  they  appeared 
to  be  upon  a  spherical  surface  which  lay  beyond  them  all.  Each 
star  would  seem  to  be  situated  where  a  straight  line  drawn  from  his 
eye  through  the  star,  met  the  spherical  surface.  This  imaginary 
spherical  surface  is  called  the  celestial  sphere. 

16.  Radius  of  the  Celestial  Sphere.  —  Astronomers  find  it  conven- 
ient to  assume  that  the  radius  of  the  celestial  sphere  is  infinite,  that 
is,  too  great  for  human  comprehension.     Were  it  possible  for  a  man 
to  take  his  stand  upon  the  celestial  sphere  and  look  back  at  the  vast 
assemblage  of  worlds  which   we  call   the  physical  universe,  their 
combined  mass  would  appear  to  him  to  be  a  mere  point  of  light, 
which  would  be  situated   at  the  centre  of  the  sphere.     Hence  it  is 
evident  that  the  observer's  eye,  or  any  point  on  the  earth's  surface, 
or  the  earth's  centre,  or  the  sun's  centre,  may  be  considered  without 
palpable  error  as  the  centre  of  the  celestial  sphere. 

17.  Revolution   of  the   Sphere.     The   Poles.  —  Imagine    that     the 
earth's  axis  is  prolonged  until  it  strikes  the  celestial  sphere  at  two 
opposite  points,  called  the  north  and  south  celestial  poles,  respect- 
ively.    The  heavens  appear  to  revolve  about  an  axis  drawn  from 
one  celestial  pole  to  the  other. 


In  Fig.  2,  P  is  the  north  pole  of  the  earth  and  F  the  south  pole ; 
O  is  the  position  of  an  observer  upon  its  surface ;  AB  is  drawn 
through  O  parallel  to  PPr.  Now  there  is  no  fixed  star  which  is 


8  DESCRIPTIVE    ASTRONOMY. 

known  to  be  nearer  to  us  than  20,000000,000000  miles.  To  give 
greater  definiteness  to  our  ideas,  conceive  the  radius  of  the  celestial 
sphere  to  be  20,000000,000000  miles,  and  its  centre  to  be  at  the 
earth's  centre.  Prolong  AB  and  PP',  until  they  strike  the  surface 
of  the  sphere :  suppose  the  places  where  they  strike  to  be  marked  by 
brilliant  points  of  light.  So  enormous  is  the  radius  of  the  sphere 
when  compared  with  the  distance  between  AB  and  PP',  that  to  an 
observer  on  the  earth  an  extremity  of  AB  would  seem  to  coincide 
with  the  corresponding  extremity  of  PP',  even  if  the  observer's  eye 
were  assisted  by  the  most  powerful  telescope  of  modern  times. 

We  may  therefore  consider  the  sky  as  revolving  on  AB  as  an 
axis,  and  may  state  the  following  principle. 

The  celestial  sphere  appears  to  revolve  on  an  axis  drawn  from  the 
eye  of  the  observer  to  either  pole  of  the  celestial  sphere. 

18.  Location  of  the  North  Celestial  Pole.  —  Nearly  every  one  is 
familiar  with  the  configuration  of  seven  bright  stars  which  is  called 


2 

N    . 

•X 


Fig.  3.  —  THE  GREAT  DIPPER  AND  POLARIS. 

"  The  Great  Dipper."  It  is  represented  in  Fig.  3.  The  two  stars 
in  the  bowl  of  the  dipper  which  point  nearly  to  the  Pole  Star 
(Polaris)  are  called  "The  Pointers."  The  distance  from  Polaris  to 
the  nearer  one  of  the  Pointers  is  about  five  times  the  distance  be- 
tween the  latter.  The  position  of  the  pole  is  shown  in  the  figure ; 


APPARENT    DAILY    MOTION    OF    THE    STARS.  9 

it  lies  very  near  a  line  drawn  from  Polaris  to  Mizar ;  its  distance 
from  Polaris  is  one  fourth  of  the  distance  between  the  Pointers. 

19.  Definitions.  —  If  a  straight  line  does  not  lie  in  a  plane,  but 
meets  it  at  some  point,  the  point  is  called  the  foot  of  the  line.  A 
straight  line  is  perpendicular  to  a  plane  when  it  is  perpendicular  to 
every  straight  line  that  can  be  drawn  in  the  plane  through  its  foot. 

The  corner  of  a  room,  where  two  walls  meet,  is  a  line  perpendic- 
ular to  the  plane  of  the  ceiling,  or  of  the  floor. 


Fig.  4. 


Fig-  5- 


A  straight  line  is  parallel  to  a  plane  when  they  cannot  meet, 
however  far  they  may  be  extended. 

When  two  planes  meet,  their  line  of  intersection  is  called  the 
edge  of  the  angle  which  they  make  with  each  other.  The  planes 
AC  and  CF  meet  in  the  edge  BC.  At  any  point,  H,  in  the  edge, 
two  perpendiculars  to  the  edge  are  drawn,  one  lying  in  each  plane. 
The  angle  GHK  made  by  these  perpendiculars  measures  the  angle 
between  the  planes. 

To  find  the  angle  which  a  line,  prolonged  if  necessary,  makes 
with  a  plane  which  it  meets,  a  per- 
pendicular to  the  plane  is  dropped 
from  some  point  A  in  the  line :  the 
foot  of  the  perpendicular  is  then 
joined  with  the  foot  of  the  original 
line.  The  angle  which  the  last  line 
drawn  makes  with  the  original  line  is  j 
the  angle  sought.  In  the  figure, 
AB  is  the  original  line,  AC  the  per- 
pendicular, BC  the  joining  line,  and  ABC  the  angle. 

20.  The  Celestial  Equator.  —  The  equator  of  the  earth  is  an  imagi- 
nary line  encircling  it,  midway  between  the  poles.  The  plane  of  the 


Fig.  6. 


10  DESCRIPTIVE    ASTRONOMY. 

equator  extended  indefinitely  cuts  the  celestial  sphere  in  a  circle, 
which  is  called  the  celestial  equator.  Since  the  plane  of  the  ter- 
restrial equator  is  perpendicular  to  the  earth's  axis,  the  plane  of 
the  celestial  equator  is  perpendicular  to  the  axis  of  the  celestial 
sphere. 

21.  The  Horizon.  —  The  word  "  horizon  "  is   commonly  used   to 
designate  the  line  where  the  earth  and  sky  appear  to  meet.     But 
astronomers  use  the  word  in  a  different  sense.     If  the  observer  holds 
a  plumb-line   so  that  it  hangs  vertically,  any  plane  surface,  like  a 
book-cover,  when  held  so  that  it  is  perpendicular  to  the  plumb-line, 
will  represent  a  portion  of  the  plane  of  the  horizon  of  the  place  of 
observation.     If  the  flat  surface  of  the  book-cover  be  extended  hori- 
zontally in  all  directions   until  it  reaches  the  sky,  the  plane  thus 
formed  is  called  the  plane  of  the  horizon.     This  plane  cuts  the 
celestial  sphere  in  a  circle  called  the  horizon. 

22.  The  Zenith,  for  any  place  on  the  earth,  is  the  point  where 
a  plumb-line  prolonged  upward  strikes  the  celestial  sphere. 

23.  The  Nadir   is    the   point   where    the    plumb-line    prolonged 
downward  strikes  the  celestial  sphere. 

The  zenith  and  nadir  are  those  points  on  the  celestial  sphere 
which  are  most  remote  from  the  horizon. 

In  the  language  of  geometry,  they  are  called  the  poles  of  the 
horizon. 

EXERCISES. 

24.  I.  Narrate    some    personal    experience    of   illusory    motion 
similar  to  those  mentioned  in  §  14. 

2.  The    diameter  of  the   earth  is    about   eight   thousand    miles. 
What  is  the    greatest   possible   distance   between  AB   and    PP'   in 
Fig.  2  ? 

3.  The  observer  is  on  the  terrestrial  equator. 

(a)  What  is  the  distance  in  miles  between  AB  and  PP'  in  Fig.  2? 

(b)  If  he   walked    toward    either   pole,   and    the   earth   were   a 
perfect  sphere,  would  this  distance  increase  or  decrease? 

4.  If  another  line  were  drawn  through  O  in  Fig.  2,  making  an 
angle  of  10°  (one  ninth  of  a  right  angle)  with  AB,  would  AB  and 
the  new  line,  when  prolonged,  meet  the  celestial  sphere  at  the  same 
points  apparently? 


APPARENT    DAILY    MOTION    OF    THE    STARS.  II 

[If  there  be  any  doubt  in  the  student's  mind  concerning  this,  he 
should  take  two  straight  sticks,  and  put  an  end  of  one  in  contact 
with  an  end  of  the  other,  so  that  the  sticks  make  an  angle  of  10° 
with  each  other.  Then,  placing  his  eye  near  the  vertex  of  the 
angle,  he  can  look  along  each  stick  at  the  sky.] 

5.  If  the  nearest  fixed  star  were   20,000000,000000   miles  from 
us,  how  many  years  would  it  take  its  light,  travelling  186,330  miles 
per  second,  to  reach  us?     Ans.  3.4+  years. 

6.  Look  at  the  Great  Dipper  and  Polaris  in  the  sky,  and  draw  a 
map  of  them ;   first  make  a  dot  for  Polaris  and  draw  a  line  below  it 
to  represent  the  horizon.     Draw  another  line  through   Polaris  per- 
pendicular  to    the    horizon    line.     Draw  the    Dipper,   showing   its 
position  with  reference  to  these  lines,  and  state  the  hour  at  which 
your  observation  was  made. 

7.  The  observer,  facing  north,  notices  that  the  Dipper  is  below 
the  pole. 

{a)  Will  the  Dipper  appear,  on  account  of  the  earth's  rotation, 
to  move  towards  his  right  hand,  or  towards  his  left? 

(#)  What  would  be  the  direction  of  motion  of  the  Dipper  if  it 
were  above  the  pole,  near  the  zenith? 

(c)  What  direction   (up  or  down),  if  at  the  right  of  the  pole? 

(d)  What  direction,  if  at  the  left? 

8.  If    a   line    be    drawn    from    your    eye    to   each    of    the    two 
"  Pointers  "  in  the  Dipper,  the  two  lines  make  an  angle  of  about  5°, 
Astronomers   commonly  say  that  the  distance  between   the  Point- 
ers is   5°.     Estimate  the  distance  from  the  Pole  Star  to  the  Pointer 
nearest  to  it. 

9.  Suppose  that  the  earth's  centre  is  fixed  in  the  centre  of  the 
celestial  sphere,  and   that  the  earth   rotates  on   its  axis.     Imagine 
all    the  stars   to  be   fixed    on  the  surface  of  the    celestial    sphere. 
If  the  earth's  axis  were  then  tipped  a  few  degrees,  would  the  north 
celestial  pole  remain  at  the  same  point  among  the  stars  as  before? 
Would  the  position  of  the  celestial  equator  be  changed  ? 

10.  If,   instead  of  tipping  the   axis  of  the  earth,  as  in  the  pre- 
ceding exercise,  the  earth  were  moved  toward  some  point  on  the 
celestial  equator  and   were    placed    1,000000  miles    away  from  its 
former  position,  the   new  direction  of  its  axis  being  parallel  to  its 
former  direction,  would   the  new  celestial  poles  appear  coincident 


12  DESCRIPTIVE    ASTRONOMY. 

with  the  former  celestial  poles?     Would   the  old  and   new  celestial 
equators  coincide? 

11.  Conceive  that  the  earth,  in   exercise  9,  is  rotating  about  a 
straight  wire  stretched  from  the  north  celestial  pole  to   the  south 
celestial  pole.     If  it  were  slid  along  this  wire  to  a  place   10,000000 
miles  from  its  first  position,  would  the  new  celestial  equator  appear 
to  coincide  with  the  old? 

12.  The   earth  makes  its  annual  journey  around    the    sun  in  a 
path  nearly  circular. 

(a)  If  the  earth's   axis  at  every  instant  during  the   year  were 
parallel  to  its  position  at  every  other  instant,  would   the  celestial 
poles  change  their  position  during  the  year? 

(b)  Would  the  celestial  equator  change  its  position? 

[The  earth's  axis  remains  almost  parallel  to  itself  during  a  year. 
Its  deviations  will  be  explained  later.  So  far  as  naked-eye  obser- 
vations are  concerned,  it  may  be  considered  as  remaining  parallel 
to  itself.] 

13.  Take  a  ball  or  an  orange  to  represent  the  earth.     Mark  on 
it    the    north    and    south    poles    and    the    equator.     Find    from    a 
geography  or  other  source  the  latitude  of  your  place  of  observation 
to  the  nearest  degree,  and  locate  the  place  on   the  ball.     (If  the 
latitude  were  45°,  the  place  would  be  halfway  between  the  pole  and 
the   equator.)     Take   a  flat  stiff  card,  and  lay  one  of  its  surfaces 
against  the  ball,  at  the  point  representing  the  place  of  observation. 
Fasten  the  card  by  a  pin  thrust  through  it  into  the  ball  at  the  point 
of  contact.     The  pin   should    point  toward  the  centre  of  the  ball. 
The  surface  of  the  card  will  then  represent  the  plane  of  the  horizon 
of  the  place,  and  will  be  tangent  to  the  spherical  surface  of  the  ball. 

(#)  Is  your  horizon  parallel  or  perpendicular  to  the  earth's 
axis? 

(&)    Is  it  parallel  to  the  earth's  equator? 

(c)  If  you  were   at  some   point  on  the   earth's  equator,  would 
your  horizon  plane  be  perpendicular  to  the  earth's  axis,  or  parallel 
to  it? 

(d)  If  you  were  at  the  north  pole,  would  your  horizon  plane  be 
parallel  to  the  earth's  axis,  or  perpendicular  to  it? 

(e)  As  the  earth  tuVns  on   its  axis,  does  the   inclination  of  the 
axis  to  the  plane  of  your  horizon  change? 


APPARENT    DAILY    MOTION    OF    THE    STARS.  13 

14.  If  the  polar  axis  of  the  ball  in  the  last  exercise  be  prolonged 
both  ways,  which  prolongation  (north   or   south)  would  pierce  the 
plane  of  the  card  representing  your  horizon  plane?     If  the  ball  be 
held  so   that  the   card  is   horizontal,  what   points   on   the   celestial 
sphere  would  the  pin  strike  if  prolonged  indefinitely  each  way? 

REMARK.  —  In  obtaining  the  answers  to  the  following  exercises, 
the  apparatus  described  in  exercise  1 3  can  be  used ;  but  it  would  be 
much  better  to  imagine  the  earth  itself,  with  its  poles  and  equator, 
and  with  horizon  planes  touching  it  at  different  points.  The  student 
should  make  every  endeavor  to  picture  to  himself  the  realities  of 
nature,  rather  than  the  apparatus  or  the  geometrical  diagrams  used 
to  explain  principles.  Wherever  possible,  he  should  observe  the  celes- 
tial motions  about  which  he  studies. 

15.  If  you  were  at  the  north  pole,  would  your  horizon  plane  be 
parallel  to  the  plane  of  the  earth's  equator,  or  perpendicular  to  it? 

If  you  lived  at  the  equator,  would  your  horizon  plane  be  parallel 
to  the  earth's  equator,  or  perpendicular  to  it? 

1 6.  (#)    If  one  man  were  at  the  north  pole  and  another  at  the 
south,  would  their  horizon  planes  be  parallel? 

(£)  If  one  man  were  at  the  north  pole  and  another  at  any  point 
of  the  equator,  would  their  horizon  planes  be  perpendicular  to  each 
other? 

(c)  Could  the  horizon  planes  at  two  points  on  the  equator  be 
parallel  ? 

(</)    Could  they  be  perpendicular  to  each  other? 

REMARK.  —  In  answering  questions  about  the  rising  and  setting 
of  the  stars,  the  student  should  remember  that  the  horizon  of  the 
place  of  observation  seems  motionless,  while  the  heavens  appear 
to  revolve  about  a  line  drawn  from  the  observer's  eye  to  either 
celestial  pole. 

17.  The  radius  of  the  celestial  sphere  is  considered  infinite. 
The  observer  is  at  the  north  pole. 

(a)  Does  his  horizon  coincide  with  the  celestial  equator? 

(b)  Is  the  north  celestial  pole  at  his  zenith? 

(c)  Could  he  see  a  star  which  lay  between  the  south  celestial 
pole  and  the  equator? 

(</)    Would  any  star  visible  to  him  set  within  24  hours? 

(e)    Every  star  in  the  sky  would  appear  to  describe  a  circle  in 


1,4  DESCRIPTIVE    ASTRONOMY. 

24  hours.     Would   the  planes  of  these  circles  be  parallel  to  his 
horizon? 

(/)  If  the  sun  were  always  north  of  the  celestial  equator,  would 
night  ever  come  for  him? 

1 8.  A  man  lives  at  some  point  on  the  equator. 

(a)  Does  his  horizon  coincide  with  the  celestial  equator? 

(b)  Do  the  celestial  poles  lie  on  his  horizon? 

(c)  Is  the  plane  of  the  celestial  equator  perpendicular  to  the 
plane  of  his  horizon? 

(d)  If  the  celestial  equator  were  drawn  as  a  line  of  light  on  the 
celestial  sphere,  would  it  pass  through  his  zenith? 

(ji)    Would  the  celestial  equator  cut  his  horizon? 

(/)    If  so,  at  what  points  (north,  south,  east,  or  west)  ? 

(^•)  How  great  a  portion  of  the  celestial  equator  would  be 
visible- at  any  instant? 

(/&)  If  a  star  rose  at  the  east  point  of  the  horizon,  at  what  point 
would  it  set? 

(i)  If  a  star  rose  a  little  north  of  the  east  point  of  the  horizon, 
would  it  set  a  little  north  of  the  west  point,  or  a  little  south  of  that 
point? 

(/)  If  a  star  rose  at  a  point  half  way  between  the  south  and 
east  points  of  the  horizon,  where  would  it  set? 

(k)    Where  would  Polaris  rise  and  set? 

(/)  For  how  many  hours  would  a  star  be  above  the  horizon, 
and  for  how  many  below? 

REMARK.  —  To  assist  in  forming  clear  ideas  about  the  answers 
to  the  questions  in  exercise  19,  the  scholar  may  take  an  orange, 
through  which  a  knitting-needle  has  been  thrust,  to  represent  the 
celestial  sphere ;  on  it  a  circle  may  be  drawn  to  represent  the  celes- 
tial equator ;  other  circles  may  be  drawn  parallel  to  this  one.  The 
orange  may  then  be  half  submerged  in  water,  as  shown  in  Fig.  67. 
The  surface  of  the  water  will  represent  the  plane  of  the  observer's 
horizon,  and  the  upper  half  of  the  orange  the  visible  heavens,  the 
observer  being  supposed  to  be  at  the  centre  of  the  orange. 

19.  The  observer  is  located  somewhere  between  the  north  pole 
and  the  equator,  say  at  40°  north  latitude. 

(a)  Is  his  horizon  plane  parallel  or  perpendicular  to  the  celestial 
equator? 


APPARENT    DAILY    MOTION    OF   THE    STARS.  15 

(b)  Does  the  axis  of  the  celestial  sphere  make  an  oblique  angle 
with  his  horizon  plane? 

(c)  Does  a  line  from  the  north  celestial  pole  to  the  observer's 
eye  make  a  right  angle  with  his  horizon  plane,  or  an  acute  angle? 

(d)  If  a  plane  be  passed  through   the   observer's  eye,  perpen- 
dicular to  the  line  last  mentioned,  will  it  be  parallel  to  the  plane  of 
the  earth's  equator? 

(e)  The  plane  just  passed  through  the  observer's  eye  intersects 
the  horizon  in  a  line :   what  direction  (north  and  south,  or  east  and 
west)  does  that  line  have  ? 

(/)  The  plane  mentioned,  when  extended  in  all  directions  to 
the  celestial  sphere,  will  cut  a  circle  on  it,  half  of  which  is  above 
the  horizon ;  if  this  semicircle  could  be  seen  as  a  line  of  light 
on  the  celestial  sphere,  would  it  lie  north  of  the  zenith,  or  south 
of  it? 

(g)  Would  this  semicircle  coincide  with  half  of  the  celestial 
equator? 

(ti)  With  extended  arm  and  forefinger  point  to  the  east  point  of 
the  horizon ;  swing  your  arm  in  such  a  way  that  your  forefinger  will 
point  to  the  celestial  equator,  as  it  runs  from  the  east  point  of  the 
horizon  to  the  west  point. 

(*)    Similarly  trace  the  circle  which  Polaris  describes  in  a  day. 

(/)  Similarly  trace  the  circle  which  a  star  10°  from  the  north 
celestial  pole  would  describe  in  a  day. 

(£)  As  seen  from  your  home,  does  the  bowl  of  the  Great  Dipper 
ever  set? 

(/)  If  a  star  rises  half  way  between  the  north  and  east  points 
of  your  horizon,  at  what  point  of  the  horizon  will  it  set? 

(m)  If  it  could  be  watched  for  24  hours,  would  it  be  above  the 
horizon  just  12  hours,  or  more,  or  less? 

(n)  If  a  star  rose  at  the  east  point  of  the  horizon,  would  it  be 
above  the  horizon  just  12  hours? 

(0)  If  a  star  rose  half  way  between  the  east  and  south  points  of 
the  horizon,  would  it  be  above  the  horizon  more  or  less  than  12 
hours? 

(/)  If  a  star  is  between  the  north  celestial  pole  and  the  celestial 
equator,  will  it  be  above  your  horizon  more  or  less  than  half  a  day 
at  a  time? 


i6 


DESCRIPTIVE    ASTRONOMY. 


(^)  If  a  star  is  between  the  south  celestial  pole  and  the  celestial 
equator,  will  it  be  below  your  horizon  more  or  less  than  half  a  day 
at  a  time  ? 

(r)    Point  your  finger  at  the  south  celestial  pole. 

(s)  Could  a  star  be  so  near  the  south  celestial  pole  that  it  could 
not  be  seen  from  your  home? 

20.  State  which  one  of  the  following  diagrams  represents  the 
apparent  daily  motion  of  the  stars  as  seen  from  the  north  pole. 
Which,  as  seen  from  the  equator.  Which,  as  seen  -from  a  place  in 
latitude  40°  north. 


Fig.  7. 


Fig.  8. 


APPARENT    DAILY    MOTION    OF    THE    STARS. 


21.  Find,  with  the  teacher's  aid  if  necessary,  some  bright  planet, 
which  will  be  visible  during  the  time  you  expect  to  devote  to  the 
study  of  this  book :   on  a  moonless  night,  make  a  map  showing  its 
position  with  reference  to  the  neighboring  bright  stars.      Preserve 
the  map,  and  note  on  it  the  planet's  position  among  the  stars  from 
week  to  week. 

22.  Take   your  seat   in   a  dark   room,  before    a    south  window. 
Adjust  your  head   so  that  by  looking  with   one  eye  just  past  the 
western  sash  of  the  window  you  will  see  a  star.     Hold  your  head 
steady  until  you  see  the  star  disappear  behind  the  sash.     The  farther 
you  are  from  the  window,  the  easier  the  observation  will  be.     If  you 
have  a  good  opera-glass  or  spy-glass,  by  fastening  it  so  that  it  will 
point  to  the  southern  portion  of  the  sky,  you  can  observe  the  motion 
of  the  stars  more  easily.     Near  the  north  celestial  pole  the  apparent 
motions  of  the  stars  are  too  slow  to   be   observed   satisfactorily  in 
this  way. 

23.  At  your  first  opportunity,  early  in  the  evening,  draw  a  map 
similar  to  that  required  in  exercise  6.     Before  retiring  for  the  night, 
look  again,  and  draw  another  map  of  the  Dipper. 

(a)  Does  a  comparison  of  these  maps  show  a  movement  of  this 
group  ? 

(£)  If  you  look  at  the  face  of  a  watch  held  between  your  eye 
and  the  north  celestial  pole,  will  its  minute  hand  move  around  in  the 
same  direction  as  the  Dipper? 


1 8  DESCRIPTIVE    ASTRONOMY. 

24.  Some  evening,  notice  the  hour  and  minute  when  some  star, 
easily  recognized  again,  is  near  the  eastern  horizon.    After  about  two 
weeks,  at  the  same  hour  and  minute,  look  for  the  star.     Is  it  nearer 
the  horizon  than  before?     Try  the  same  experiment,  at  the  same 
time,  with  a  star  near  the  western  horizon.     From  these  observations 
determine  whether  a  star  will  rise  and  set  earlier  than  at  present,  a 
month  from  now,  or  later. 

25.  Find   from   an   almanac   or  diary  the   date   of  the  next  new 
moon.    An  evening  or  two  thereafter,  look  in  the  west  for  the  moon, 
in  the  evening  twilight.     When   first  seen,  draw  a  sketch  of  it  and 
date  the  sketch.     Sketch  its  form  every  clear  evening  thereafter  until 
it  does  not  rise  before  your  bedtime  :   then  look  for  it  in  the  morning, 
and  continue  sketching  it,  if  possible,  until  the  next  new  moon. 

(a)    While  making  these  sketches,  did  you  notice  that  the  moon 
moved  westward  among  the  stars  ? 

(#)    Did  you  see  the  dark  part  of  the  moon? 

(c)  When   the  moon  was   a  slender  crescent  did   the   cusps   or 
"  horns"  of  the  crescent  point  toward  the  sun? 

(d)  When  the  moon  was  full  (a  complete  circle  of  light)  did  it 
rise  at  about  the  time  of  sunset? 

(^)    Did  you  ever  see  a  star  between  the  cusps  of  the  moon? 
(/)    Did  you  ever  see  the  moon  occult  a  star,  that  is,  hide  it  from 
view? 

26.  On  some   moonless   night,  find  the  Milky  Way,  which  is  a 
broad  band  of  hazy  light. 

(a)    Are  there  any  dark  places  in  it? 
(#)    Are  there  any  brilliant  spots  in  it? 
(c)    Does  it  run  through  the  Great  Dipper? 


THE    TELESCOPE.  19 


CHAPTER   III. 

THE   TELESCOPE.1 

"  Through  thee  will  Holy  Science,  putting  off 
Earth's  dusty  sandals  from  her  radiant  feet, 
Survey  God's  beauteous  firmament  unrolled 
Like  to  a  book  new-writ  in  golden  words, 
And  turn  the  azure  scroll  with  reverent  hand, 
And  read  to  men  the  wonders  God  hath  wrought." 

ANON. 

25.  Refractors  and  Reflectors.  — There  are  two  kinds  of  telescopes, 
called  respectively  refractors  and  reflectors.     Opera-glasses  and  spy- 
glasses belong  to  the  former  class. 

Reflectors  are  rarely  seen,  except  in  connection  with  astronom- 
ical observatories.  The  principal  portion  of  one  of  these  is  a  large 
curved  mirror,  which  reflects  the  rays  of  light  coming  from  the 
object  viewed,  in  a  manner  to  be  explained  hereafter. 

In  order  to   understand  the   action  of  a  telescope,  one  must  be 
acquainted  with  a  few  elementary  principles 
of  optics,  which  we  proceed  to  unfold. 

26.  Reflection    by    a    Plane    Mirror.  —  In 
the  figure,  DC  is  a  ray  of  light  striking  the 
plane  mirror  AB  at  the  point  C ;    it  is  re- 
flected along  the  line  CF.     EC  is  perpen- 
dicular to  AB.     The  angle  DCE,  which  the    A  \I7  ^  n 
incident  ray  makes  with  the  perpendicular                     C 

to  the  mirror  at  C,  the  point  of  incidence,    Fi§-  10.  — REFLECTION  BY  A 

,j  /         r    •      •  j  r--      -i      i       ^  PLANE  MIRROR. 

is  the  angle  of  incidence.      Similarly  ECF 

is  the  angle  of  reflection.     The  angle  of  incidence  is  equal  to  the  angle 

of  reflection.  - 

1  It  is  well  to  illustrate  this  chapter  as  thoroughly  as  possible  by  experiments  with 
lenses  and  mirrors.  When  a  class  is  pressed  for  time,  the  more  mathematical  portions 
of  the  chapter  may  be  omitted,  or  simply  explained  by  the  teacher.  If  an  equatorially 
mounted  telescope  is  not  available,  a  rude  wooden  model  of  it  may  easily  be  made,  and 
will  be  of  service. 


20 


DESCRIPTIVE    ASTRONOMY. 


27.  Reflection  by  a  Concave  Mirror,  —  A  concave  mirror  may  be 
considered  as  made  up  of  a  very  large  number  of  minute  plane 
mirrors.  If  a  system  of  parallel  rays  strikes  the  surface  of  a  con- 
cave spherical  mirror,  each  ray  will  be  reflected  at  its  point  of 
incidence,  in  accordance  with  the  principle  of  §  26.  The  reflected 
rays  will  converge  and  will  meet  (almost  exactly)  at  a  point  called 
the  focus  (Fig.  n).  Opticians  make  their  mirrors  deviate  slightly 
from  a  true  spherical  form,  in  such  a  way  that  all  rays  are  brought 
accurately  to  a  focus. 


NOTE.  —  In 
in  §§  37,  38. 


28-34  the  dispersion  of   light  is  neglected  ;   it  is  explained 


Air 


FOCU3 


Fig.  ii.  —  REFLECTION  BY  A  CONCAVE 
MIRROR. 


Air 


Fis:.  12.  — REFRACTION. 


28.  Refraction  by  a  Prism-  —  A  ray  of  light  passing  from  one 
medium,  as  air,  into  another  of  a  different  density,  as  glass,  is  bent 
out  of  its  course,  unless  it  strikes  the  surface  of  the  second  medium 

perpendicularly.     This  bend- 
ing is  called  refraction. 

The  ray  AB,  striking  ob- 
liquely on  the  surface  XY  of 
the  glass  prism  XYZ,  is  re- 
fracted and  travels  along  BC  ; 
at  emergence,  the  ray  is  again 
refracted,  taking  the  direction 
CD. 

Fig.  13. -REFRACTION  BY  PRISMS.  29'     Acti°n      °f     a     Number 

of  Prisms.  —  By  inspection  of 

Fig.  13  we  see  that  a  number  of  pieces  of  glass  may  be  so  arranged 
that  a  system  of  parallel  rays,  in  the  plane  of  the  paper,  falling  upon 


THE    TELESCOPE. 


21 


them  will  be  converged  to  a  common  point.  If  the  number  of 
pieces  of  glass  be  largely  increased,  each  piece  being  very  small,  the 
broken  lines  ABC  and  ADC  will  approach  closely  to  arcs  of  circles. 
Hence,  if  a  single  piece  of  glass  be  so  shaped  that  ABC  and  ADC 
will  be  nearly  arcs  of  circles,  the  system  of  rays  will  be  converged, 
as  above,  to  a  single  point,  called  the  focus. 

30.  Lenses.  —  A  common  burning  glass  is  circular  in  form,  and 
when  looked  at  edgewise  has  the  shape  of  (#)  in  Fig.  14.  The  two 
surfaces  of  the  glass  are  portions  of  spherical  surfaces.  The  sun's 


(b) 


Fig.  14.  —  LENSES. 


Fig.  15.  —  RAYS  MADE  DIVERGENT. 


rays  striking  on  the  glass  are  converged  to  a  focus.  Such  a  glass 
is  called  a  double  convex  lens:  ($),  one  side  of  which  is  flat,  is  a 
plano-convex  lens ;  (c)  is  double  concave ;  {d}  is  plano-concave ; 
(c)  and  (y),  instead  of  bringing  a  system  of  parallel  rays  to  a  focus, 
make  them  diverge  as  shown  in  Fig.  15. 


B 


Fig.  16.  — VISUAL  ANGLE. 


31.   Visual  Angle.  —  The  visual  angle  of  an  object  is  the  angle 
made  by  two  lines   drawn  from  the   eye  to   the   e.  vtrpp  jtjfnf  the 


UNIVEBSITY 


22 


DESCRIPTIVE    ASTRONOMY. 


object.  In  Fig.  16,  the  object  AB  is  placed  in  three  different 
positions  AB,  A'  B',  and  A"  B",  the  eye  being  at  E.  The  visual 
angles  are  respectively  AEB,  A'EB',  and  A"EB".  The  nearer 
the  object  is  to  the  eye,  the  greater  is  the  visual  angle,  and  the 
larger  the  object  appears.  If  the  object  be  carried  away  from  the 
eye,  the  visual  angle  will  become  less,  and  the  object  will  appear 
smaller. 


Fig.  17.  — OBJECT  AND  IMAGE. 

32.  Formation   of   an   Image.  —  The   action  of  a   double  convex 
lens   in   forming  an  image   of  an  object   may  be   easily  seen  in  a 
photographic  camera.     Let  the  arrow  in   Fig.  17  represent  a  tree 
which  is  to  be  photographed.     From  the  point  of  the  arrow  come 
innumerable  rays  of  light,  which  strike  the  outer  face  of  the  lens, 
and   are   refracted  by  it  to  a  common   focus  at  A.     Similarly  the 
rays  from  the  other  end  of  the  arrow  are  brought  to  a   focus  at  B. 
When  the  photographer  adjusts  his  ground-glass  so  that  the  points 
A  and  B  lie  on   its  surface,  an   image  of  each   of  these   points  is 
formed  on  the  glass,     Every  other  point  of  the  arrow  images   itself 
on  the  glass,  in  like  manner.     The  object-glass  of  a  telescope  is  the 
large  lens  at  the  end  farthest  from  the  eye :    its  function  is,  like  the 
photographer's  lens,  to  form  an  image  of  the  object  viewed. 

33.  Action  of  the  Eyepiece.  —  In  a   telescope  there  is  no   ground- 
glass  to  receive  the  image  formed  by  the  object-glass,  but  the  rays 
of  light  pass  on  through  another  lens,  called   the   eyepiece,  before 
reaching  the  eye.     The  three  rays  shown  in  Fig.   18  as  diverging 
from  the  point  A  in  the   image  are   rendered   more  nearly  parallel 


THE    TELESCOPE. 


to  each  other  in  passing  through  the  eyepiece,  and  are  sharply 
bent.  The  rays  diverging  from  B  are  bent  in  the  same  .fashion. 
Prolong  the  central  one  of  the  rays  coming  from  A  backward  to 
any  convenient  point,  Y,  and  the  central  ray  from  B  to  X,  and  draw 
the  arrow  XY.  AB  seen 

through     the     eyepiece  crE  PIECE  -~-«X 

looks  as  large  as  XY 
would  without  the  eye- 
piece, XEY  being  the 
common  visual  angle. 

34.    A  Simple  Refract- 
ing   Telescope.  —  Placing 
the  object-glass  and  eye- 
piece   at    opposite    ends  Fig.  iS.  —  LENS  AS  EYEPIECE. 
of   a   tube,    we    have    a 

telescope.  In  Fig.  19,  suppose  the  telescope  to  be  pointed  at  a 
distant  tree :  the  rays  A',  A,  A",  coming  from  the  top  of  the  tree, 
meet  at  the  focus  a,  and,  passing  on  through  the  eyepiece,  enter  the 
observer's  eye.  The  rays  B',  B,  B",  coming  from  the  bottom  of  the 


Fig.  19. — OBJECT-GLASS  AND  EYEPIECE. 

tree,  meet  at  the  focus  b,  and  pass  on  through  the  eyepiece  to  enter 
the  observer's  eye.  The  central  rays,  A  and  B,  of  the  two  systems 
meet  at  E,  and  to  the  observer  the  tree  appears  to  subtend  a  visual 
angle  b'E  a'.  If  the  object-glass  be  now  removed,  and  the  observer, 
placing  his  eye  at  O,  where  the  centre  of  the  object-glass  was,  looks 
at  the  tree,  it  will  subtend  a  -visual  angle  of  AOB.  If  he  were  to 
stand  near  the  eyepiece,  the  visual  angle  would  be  a  trifle  smaller 
than  AOB,  since  he  is  a  little  farther  from  the  tree.  But  when 
he  looked  through  the  telescope,  the  tree  appeared  to  subtend  the 


24  DESCRIPTIVE    ASTRONOMY. 

angle  b'E  a',  which  is  much  larger  than  AOB.     This  explains  why 
a  telescope  magnifies  an  object. 

35.  Object-glasses  of  Various  Sizes.  —  When  one  looks  at  a  star,  only 
those  rays  which  fall  upon  the  pupil  enter  the  eye.     Were  the  pupil 
larger,  more  rays  would   enter,  and    the  star  would    appear  more 
brilliant.     If  the   area    of  the    object-glass   of  a   telescope    is    one 
hundred  times  the  area  of  the  pupil  of  the  eye,  one  hundred  times 
as  much  light  will  fall  upon  it  as  upon  the  unaided  eye.     The  pupil 
of  a  human  eye,  when  not  exposed  to  a  bright  light,  has  on  the 
average  a  diameter  of  one  fifth  of  an  inch.     An  object-glass  one  inch 
in  aperture  (as  its  diameter   is  called)  has  a  diameter  five  times  as 
great  as  that  of  the  pupil  of  the  eye.     A  twenty-inch   object-glass 
has  a  diameter  one  hundred  times  as  great.     Geometry  teaches  that 
the  areas  of  two  circles   are  to  each  other  as  the  squares  of  their 
diameters :   hence  a  one-inch   object-glass  collects  not  merely  five 
times  as  much  light  as  the  pupil  of  the  eye,  but  52,  or  25  times  as 
much.     A  twenty-inch  "  objective"  collects  ioo2,  or  10,000  times  as 
much    light   from    any  star,    as    the    unassisted    eye    does.     About 
1 8  per  cent  of  this  light  is  lost  in  passing  through  the  object-glass 
and  eyepiece. 

36.  Magnifying  Power  of  Eyepieces.  —  After  the  object-glass   has 
made  a   brilliant   image  of  an  object,   at   the  focus,   the    eyepiece 
magnifies  the  image,    as  shown   in    §   34.     By  using  lenses  of  dif- 
ferent  degrees    of  curvature,   different    "  magnifying    powers "    are 
obtained. 

If  the  apparent  diameter  of  the  planet  Jupiter,  for  instance,  were 
increased  sixteen  fold  by  a  telescope  armed  with  a  certain  eyepiece, 
the  magnifying  power  of  that  eyepiece  would  be  sixteen  diameters. 
Very  high  magnifying  powers  cannot  be  used  advantageously,  be- 
cause of  the  disturbances  continually  going  on  in  our  atmosphere. 
The  rays  of  light  from  a  star,  coming  through  various  disturbed 
strata  of  air  of  different  densities,  are  bent  hither  and  thither,  so  that 
the  image  of  the  star  in  the  telescope  dances  about,  and  looks 
blurred.  The  higher  the  magnifying  power  employed,  the  worse 
the  blur.  Ordinarily,  an  eyepiece  the  magnifying  power  of  which 
is  more  than  twenty  times  the  aperture  of  the  object-glass  (in  inches), 
cannot  be  used  to  advantage.  But  when  the  atmosphere  is  exceed- 
ingly calm,  a  power  of  one  hundred  times  the  aperture  may  be  used 


THE    TELESCOPE. 


25 


on  a  bright  object.     A  glass  six  inches  in  aperture  would  then  bear 
a  power  of  six  hundred  diameters. 

37.  Dispersion  of  Light.  —  When  a  beam  of  sunlight  is  passed 
through  a  prism,  and  then  allowed  to  fall  on  a  screen,  a  colored 
spot  is  seen  where  the  light  strikes  the  screen.  The  spot  is  red  at 


Fig.  20.  —  DISPERSION, 


one  end  and  violet  at  the  other.  Careful  experiments  show  that 
sunlight,  when  passed  through  a  prism,  is  decomposed  into  the 
following  colors :  red,  orange,  yellow,  green,  cyan-blue,  ultra- 
marine blue,  and  violet.  The  light  thus  decom- 
posed is  said  to  be  dispersed.  The  figure  shows 
that  the  red  rays  are  not  refracted  (deviated  from 
their  original  direction)  as  much  as  the  violet  rays. 
A  prism  of  flint  glass  separates  the  red  rays  from 
the  violet  more  widely  than  a  prism  of  crown 
glass. 

38.  Correction  of  Dispersion.  —  The  dispersion 
caused  by  one  prism  may  be  counteracted  by 
passing  the  dispersed  beam  through  a  similar  in- 
verted prism.  The  final  emergent  beam  is  parallel 
to  the  original  beam.  By  passing  a  beam  through  Fig'  ^.-DISPERSION 

CORRECTED. 

two  prisms  of  different  angles,  one  being  of  crown 

glass,  and  the  other  of  flint,  it  is  possible  to  correct  the  dispersion 

very  nearly,   and  at  the  same  time  to   alter  the  direction  of  the 

beam. 


20  DESCRIPTIVE    ASTRONOMY. 

39.  An  Achromatic  Object-glass.  —  This  is  shown  in  Fig.  22.  Al- 
most all  object-glasses  are  made  of  two  lenses :  the  outer  lens  is  of 
crown  glass,  and  is  double  convex ;  the  inner  is  of  flint  glass,  and  is 


Fig.  22.  —  ACHROMATIC  OBJECT-GLASS. 

nearly  plano-concave.     Such  an  object-glass  is  said  to  be  achromatic 
(without  color) .     The  largest  and  finest  object-glasses  in  the  world 

have  been  ground  by  the  firm 
of  Alvan  Clark  and  Sons  of 
Cambridge,  Mass.  In  1881, 
Prof.  Abbe  and  Dr.  Schott,  of 
Jena,  Germany,  began  a  series 
of  experiments  which  have  re- 
sulted in  the  manufacture  of 
lenses  of  various  refractive  and 
dispersive  powers,  combina- 
tions of  which  give  almost  per- 
fect achromatism.  Some  of 
these  lenses,  however,  tarnish 
in  time.1 

40.  Achromatic  Eyepieces.  — 
Eyepieces  are  made  achro- 
matic also,  for  a  bad  eyepiece 
undoes  the  work  of  a  good 
object-glass.  There  are  two 
common  forms,  the  Huygheni- 

Fig.  23.— ALVAN  CLARK. 

an,  or  negative,  and  the  Rams- 
den,  or  positive :  the  former  is  more  achromatic  than  the  latter. 
The  large  lens  of  the  negative  eyepiece  receives  the  rays  from  the 


1  Mr.  J.  A.  Brashear,  the  well  known  optician,  of  Allegheny,  Pa.,  makes  a  specialty 
of  grinding  lenses  of  the  new  glass. 


THE    TELESCOPE. 


object-glass  just  before  they  come  to  a  focus;   the  focus  is  formed 
between  the  two  lenses  of  the  eyepiece.     In  the  case  of  the  positive 


NEGATIVE  POSITIVE 

Fig.  24. —  EYEPIECES. 

eyepiece,  the  rays  come  to  a  focus  before  they  reach  the  eyepiece. 
A  positive  eyepiece  can  be  used  as  a  hand  magnifying  glass,  but  a 
negative  cannot. 

41.  The  Reflector.  —  A  reflector,  or  reflecting  telescope,  receives 
its  name  from  the  fact  that,  instead  of  an  object-glass,  it  has  a  large 
concave  mirror,  which  reflects  to  a  focus  the  rays  from  a  celestial 


EYEPIECE 

Fig.  25. —  PATH  OF  RAYS  OF  LIGHT  IN  A  REFLECTOR. 
I 

object.  Reflectors  have  many  forms,  differing  in  minor  particulars. 
The  form  most  used  is  the  Newtonian,  devised  by  Sir  Isaac  Newton, 
which  is  shown  in  Fig.  26. 

The  mirror  is  now  made  of  glass,  on  which  a  thin  film  of  silver  is 
deposited  by  a  chemical  process. 

42.  Comparison  of  Refractors  and  Reflectors.  —  The  silver  makes  a 
brilliant  reflecting  surface,  and  there  is  no  trouble  from  dispersion 
of  light,  as  in  the  case  of  a  refractor.  This  gives  a  reflector  an 
advantage  for  photographic  and  spectroscopic  work.  But  a  large 
reflector  has  many.disadvantages  when  compared  with  a  refractor 
of  equal  power.  The  silver  film  tarnishes,  and  must  be  renewed 
periodically.  Slight  deformations  of  the  concave  surface,  caused  by 


28 


DESCRIPTIVE    ASTRONOMY. 


Fig.  26.  —  A  NEWTONIAN  REFLECTOR  MADE  BY  BRASHEAR. 


THE    TELESCOPE. 


Fig.  27.  —  LORD  ROSSE'S  SIX-FOOT  REFLECTOR 

(From  Scribner's  Magazine,  by  permission.) 


3O  DESCRIPTIVE    ASTRONOMY. 

a  difference  in  temperature  between  the  front  and  the  back  of  the 
mirror,  or  by  sagging  because  of  its  own  weight,  distort  the  image 
of  the  object  looked  at. 

43.  Some   Noted   Reflectors.  —  Lord    Rosse   has    at    Parsonstown, 
Ireland,    the  largest  reflector  ever  built.     The  mirror,  which  con- 
sists of  an  alloy  of  copper  and  tin,  is  six  feet  in  diameter.     Mr. 
A.  A.  Common,  a  wealthy  English  amateur,  has  made  a  silver  on 
glass   reflector,   sixty-two    inches    in  aperture,   which    is  more  effi- 
cient  than    any  other   ever   constructed.      Such    an   instrument   is 
capable   of  marvellous  work   in    photographing    faint    objects    like 
nebulae.     Fig.  27  represents  Lord  Rosse's  six-foot  reflector.     There 
is  no  reflector  in  America  which  approaches  these  in  power. 

44.  An  Equatorial  Mounting. — Though  a  large  telescope  be  well- 
nigh  perfect   optically,    it   will   be    practically   useless    unless   well 


W 


]D 


Fig.  28.  —  SCHEME  OF  AN  EQUATORIAL  MOUNTING. 

mounted.  On  account  of  the  diurnal  motion  of  the  stars,  if  the 
telescope  is  pointed  at  one,  and  is  motionless,  the  star  will  quickly 
pass  out  of  the  field  of  view.  The  best  mounting,  when  an  object  is 
to  be  watched  for  some  time,  is  therefore  one  which  will  enable  the 
telescope  to  follow  the  object  most  easily.  It  has  been  shown  (§17) 
that  the  heavens  appear  to  rotate  daily  about  an  axis  drawn  from 
the  north  celestial  pole  through  the  observer.  Let  a  strong  steel 
axis,  AB,  be  supported  at  each  end  on  pivots  A  and  B,  on  which  it 
may  turn,  and  let  it  point  to  the  north  celestial  pole.  This  steel 
axis  may  then  be  considered  as  a  portion  of  the  axis  about  which 
the  celestial  sphere  appears  to  rotate.  Through  the  hole  O  let 


THE    TELESCOPE.  3! 

the  axis  CD,  to  which  the  telescope  is  fastened,  be  thrust,  and 
the  weight  W  be  placed  at  C,  to  balance  the  telescope.  Let  the 
telescope  be  pointed  to  any  star,  and  the  axis  AB  be  turned  by 
suitable  clockwork,  so  that  it  will  make  one  revolution  in  twenty- 
four  hours.  Then  the  telescope  will  move  just  as  it  would  if  it  were 
actually  fastened  to  the  axis  of  the  celestial  sphere,  and  will  con- 
tinue to  point  to  the  star. 

45.  Illustration  of  the  Principle  of  an  Equatorial   Mounting.  —  To 
get  a  clearer  idea  of  this,  one  may  imagine  that  the  earth's  axis  is  a 
wooden  pole  running  through  it ;  further,  that  a  person  in  the  interior 
of  the  earth  nails  a  lath  to  this  pole  in  such  a  way  that  the  lath 
points  to  the  city  of  Boston.     The  lath  will  continue  to  point  to 
Boston  as  the  earth  rotates.     If  the  lath  had  originally  pointed  to 
any  other  city,  it  would  continue  to  do  so,  whether  the  city  were 
near  the  equator  or  in  the  vicinity  of  one  of  the  poles.     The  lath 
represents  the  telescope,  and  the  city,  a  star. 

46.  The  German  Form  of  an  Equatorial  Mounting,  —  The   form   of 
mounting  just  described  is  not  suited  to 

a  large  refractor,  because  the  axis  AB 
would  necessarily  be  very  large  and 
cumbrous.  In  Fig.  29,  AB  is  parallel 
to  the  earth's  axis,  and  is  called  the 
polar  axis.  CD,  the  declination  axis 
(§122),  runs  through  the  "sleeve" 
EF,  which  is  bolted  to  the  top  of  the 
polar  axis.  The  telescope  tube  is  fas- 
tened  to  the  declination  axis.  W  is  a 

Counterpoise,    to    balance    the   weight   of   Fig.  29.  —  A  GERMAN  EQUATORIAL. 

the  telescope,  so  that  the  whole  mechan- 
ism will  be  nicely  poised  on  the  polar  axis.      The  German  form 
of  mounting  is  shown  in  Fig.  30. 

47.  Management  of  a  Telescope.  —  The  following  directions  are  for 
inexperienced  observers  using  small  telescopes. 

1.  Do  not  look  through  a  closed  window;   the  irregularities  of 
the  pane  of  glass  will  distort  objects. 

2.  Do  not  point  the  telescope  out  of  an  open  window,  in  a  room 
warmer  than  the  external  air ;   the  warm  air  currents  rushing  from 
the  room  cause  the  images  of  objects  to  waver. 


DESCRIPTIVE    ASTRONOMY. 


Fig.  30.  —  THE  LICK  TELESCOPE. 


THE    TELESCOPE.  33 

3.  If  a  telescope  has  several  eyepieces,  use   the   one  of  lowest 
power  first;   if  the  object  appears  distinct,  you  may  try  a  higher 
power. 

4.  For  comets  and  nebulae  use  low  powers. 

5.  In    general,    avoid    touching    the    telescope    when    looking 
through ;   you  may  shake  it. 

6.  Focus  carefully,  by  sliding  the  eyepiece  in  or  out,  until  the 
view  is   most  distinct.     Different  eyes    frequently  require  different 
adjustments  of  the  eyepiece. 

7.  Clean  the  object-glass  rarely,  and  carefully.     A  little  dust  on 
it  will  be  of  no  appreciable  detriment.     Dust  may  be  removed  with 
a  camel's-hair  brush.     Never  rub  the  glass  with  any  material. 

8.  Keep  the  eyepieces  clean. 

EXERCISES. 

48.    i.  If  an  incident  ray  be  perpendicular  to  the   surface  of  a 
plane  mirror,  what  direction  will  the  reflected  ray  take? 

2.  Is  the  image  on  the  ground-glass  of  a  photographer's  camera 
upright  or   inverted?     Does    an    object  which    is    at   the    right   of 
another,  as  seen  with  the  naked  eye,  appear  at  the  right  of  it  on  the 
ground-glass? 

3.  If  you  look  through  the  wrong  end  of  a  telescope,  are  objects 
magnified,  or  are  they  minified? 

4.  The  object-glass  of  the  Lick  telescope  is  36  inches  in  diameter. 
If  the  diameter  of  the  pupil  of  one's  eye  be  one  fifth  of  an  inch,  how 
many  times  as  much  light  as  the  unaided  eye  does  the  Lick  glass 
receive  from  a  star? 

5.  When  looking  across  a  landscape   at  a  distant  fixed  object, 
did  you  ever  notice  that  the  object  trembled  slightly,  or  had  a  wavy 
appearance? 

Explain  the  cause  of  this  appearance. 

6.  Draw  a  circle  having  a  radius  of  two  inches.     With  the  same 
centre  draw  another  having  a  radius  of  2\  inches.     At  some  point 
of  the  inner  circle  draw  a  line  about  three  inches  long,  tangent  to 
it.     From  the  same  point  draw  outward  a  perpendicular  to  the  tan- 
gent, and  two  oblique  lines,  making  respectively  angles  of  45°  and 
10°  with  the  tangent.     The  smaller  circle  represents  the  earth,  and 

3 


34  •         DESCRIPTIVE    ASTRONOMY. 

the  space  between  the  circles  the  atmosphere.  The  tangent  is  the 
observer's  horizon.  When  a  man  looks  straight  up  at  the  heavens, 
does  he  look  through  more  atmosphere,  or  less,  than  when  he  looks 
nearly  horizontally? 

7.  On  a  given  night  does  the  moon  appear  more  brilliant  when 
in  mid-heaven  than  when  rising  or  setting?     Give  a  reason  for  your 
reply. 

8.  On  a  clear,  moonless  night,  do  stars  near  the  zenith   twinkle 
more  violently,  or  less,  than  those  near  the  horizon?     Why? 

9.  Did  you  ever  see  a  fixed  star  set?     Would  the  observation  of 
its  setting  be  difficult  at  sea? 

10.  Which  rays  are  the  more  refrangible,  the  red  or  the  violet? 
IT.    When    a   person    is    looking   through    a    telescope,    if   you 

hold  your  finger  in  front  of  the  object-glass  and  near  it,  will  he 
see  it  ? 

12.  If  a  person  were   looking  through  a  telescope  at  the   full 
moon,  and  another  suddenly  covered  up  one  half  of  the  object-glass, 
how  would  the  appearance  of  the  moon  be  changed? 

13.  In  an  equatorial  mounting,  what  angle  does  the  declination 
axis  make  with  the  sight  line?     What  angle  does  the   declination 
axis  make  with  the  polar  axis  ? 

14.  If  one  takes  hold  of  the  telescope  at  the  eye  end  and  moves 
it  about  its  axes  into  various  positions,  will  the  angle  between  the 
declination  axis  and  the  sight  line  change  ?     Will  the  angle  between 
the  declination  axis  and  the  polar  axis  change? 

15.  An  astronomer  using  an  equatorial  (as  the  whole  instrument 
is  usually  designated)  in  a  fixed  observatory  points  the  telescope  to 
various  parts  of  the  sky. 

(a)  Does  the  polar  axis  turn? 

(b)  Does  the  polar  axis  point  to  different  points  on  the  celestial 
sphere? 

(c)  Does  the  declination  axis  point  toward   different  points  on 
the  celestial  sphere? 

1 6.  (a)    Does  the  declination  axis  ever   lie   in  a  plane  perpen- 
dicular to  the  polar  axis? 

(b)  Does  the  declination  axis  always  lie  in  a  plane  perpendicular 
to  the  polar  axis? 

(c)  If  the   polar  axis   be   turned  through    one   revolution,  what 


THE    TELESCOPE.  35 

circle  would  the  declination  axis,  prolonged  to  the  celestial  sphere, 
trace  on  it? 

REMARK.  —  In  answering  the  following  exercises,  scholars  may 
obtain  assistance  by  using  a  pair  of  shears.  Let  the  cutting  edge 
of  one  blade  represent  the  sight  line,  the  edge  of  the  other  the  polar 
axis,  and  the  rivet  holding  the  blades  together  the  declination  axis. 
The  shears  should  be  so  held  in  the  hand  that  the  blade  representing 
the  polar  axis  points  to  the  north  celestial  pole. 

17.  If   the    sight   line  of  a  telescope   equatorially  mounted   be 
placed  (by  turning  the  telescope  about  the  declination  axis)  perpen- 
dicular to  the  polar  axis   (see  Fig.  28),  it   will   be  parallel  to   the 
plane  of  a  well  known  circle.     What  is  the  name  of  the  circle? 

1 8.  When   the    sight  line  of  an   equatorial   telescope   has   been 
placed  perpendicular  to  the  polar  axis,  if  the  latter  be  rotated  by 
the  clockwork,  what  circle  will  the  sight  line,  prolonged  to  the  celes- 
tial sphere,  trace  upon  it? 

19.  If  the  sight  line  of  an  equatorial  be  placed  parallel   to  the 
polar  axis,  toward  what  point  on  the  celestial  sphere  will  the  tele- 
scope point? 

20.  The  telescope,  placed  as  in  the  preceding  exercise,  is  rotated 
on  its  polar  axis. 

(a)   Will  the  sight  line  continue  to  be  parallel  to  the  polar  axis? 
(#)    What  sort  of  a  geometrical  figure  will   the  sight  line,  pro- 
longed, trace  on  the  celestial  sphere? 

(c)  Would  that  figure,  if  it  could  be  seen  from  the  earth,  appear 
large  or  small  ? 

21.  If  the  sight  line  of  an  equatorial  be   placed   at   an   oblique 
angle  with  the   polar  axis,  and  the  instrument  be  rotated  on  that 
axis,  will  the  sight  line  describe  a  circle  on  the  celestial  sphere? 

22.  Do  stars  near  the  celestial  equator  seem  to  move  across  the 
sky  more  swiftly  than  those  near  the  pole,  or   more   slowly?     Will 
the  clockwork  of  an  equatorial  (when  so  rated  that  a  star  near  the 
celestial  equator  will  be  kept  in  the  field  of  the  telescope)  need  a 
special  adjustment,  to  enable  it  to  keep  a  star  near  the  pole  in  the 
field? 


36  DESCRIPTIVE    ASTRONOMY. 


CHAPTER    IV. 

THE    SUN. 

x 

"  But  yonder  comes  the  powerful  king  of  day, 
Rejoicing  in  the  east.     The  lessening  cloud, 
The  kindling  azure,  and  the  mountain's  brow, 
Illumed  with  fluid  gold,  his  near  approach 
Betoken  glad." 

THOMSON. 

49.  Distance  and  Diameter.  —  The  average  distance  of  the  earth 
from  the  sun  is  92,900000  miles. 

Prof.  Mendenhall  has  said  that,  if  a  babe  could  instantaneously 
reach  across  this  stupendous  gulf  and  touch  the  glowing  surface  of 
the  sun,  he  would  never  realize  that  his  hand  was  burned,  for,  though 
the  nerves  transmit  sensations  to  the  brain  ^ith  great  rapidity,  over 
one  hundred  years  would  be  required  for  this  message. 

The  sun's  diameter  is  866,500  miles,  109.5  times  as  great  as  that 
of  the  earth.  Were  the  earth  to  swell  to  the  size  of  the  sun,  and  were 
men  to  increase  in  the  same  ratio,  an  average  man  would  be  625  feet 
tall.  Since  he  would  also  be  109.5  times  as  broad  and  109.5  times  as 
thick  as  at  present,  his  bulk  would  become  109.5  X  IO9-5  X  IO9-5»  or 
more  than  1,300000  times  as  great  as  now. 

The  force  of  gravity  at  the  sun's  surface  being  27.6  times  as 
powerful  as  at  the  surface  of  the  earth,  our  giant,  if  transported  to 
the  sun,  would  weigh  2,750000  tons,  if  of  ordinary  build. 

50.  How  to  View  the  Sun  through  a  Small  Telescope.  —  The   sun 
may  be  viewed,  for  a  moment,  through  a  pinhole  in  a  card,  without 
injury  to  the  eye,  but  the  observation  is  not  to  be  recommended. 

A  small  telescope  may  be  directed  toward  the  sun  by  moving  it 
until  its  shadow,  thrown  on  a  book  held  near  the  lower  end,  is  as 
small  as  it  can  be  made,  and  a  dazzling  light  issues  from  the  eye- 
piece. A  dark  shade  glass  may  then  be  held  close  to  the  eyepiece, 
and  one  may  look  through.  Great  care  should  be  taken  to  avoid 
getting  the  full  blaze  of  sunlight  into  the  eye,  and  one  should  not 


THE    SUN. 


77 


continue  looking  through  a  dark  glass,  if  the  sun  is  uncomfortably 
bright. 

The  dark  glass  and  eyepiece  soon  become  hot,  and  may  break  if 
the  telescope  is  kept  pointed  at  the  sun  too  long.  The  amount  of 
light  and  heat  may  be  much  diminished  by  covering  the  object-glass 
with  a  piece  of  paper  having  a  circle  an  inch  or  more  in  diameter 
cut  in  it,  but  one  will  not  be  able  to  see  the  more  delicate  details  of 
the  sun's  surface  quite  as  well. 

51.  Use  of  a  Screen.  —  If  a  piece  of  white  paper  be  held  about  a 
foot  from  the  eyepiece,  and  the  eyepiece  pulled  out  a  fraction  of  an 
inch  beyond  its  proper  focal  position,  an  image  of  the  sun  will  be 
formed  on  the  screen.  By  careful  adjustment  of  the  eyepiece,  the 
sun  spots  will  usually  be  well  seen. 


Fig.  31.  —  THE  SUN'S  IMAGE  ON  A  SCREEN 


Fig.  32. —  ABSORPTION  OF  LIGHT 
BY  THE  SUN'S  ATMOSPHERE. 


Another  screen  fastened  to  the  telescope,  as  shown  in  Fig.  31, 
so  as  to  throw  its  shadow  on  the  first  screen,  will  improve  the  view. 
Several  persons  can  thus  view  the  sun  at  once.  The  telescope,  unless 
equatorially  mounted  and  driven  by  clockwork,  must  be  moved 
about  every  minute  in  order  to  keep  the  sun's  image  on  the  screen. 
If  the  eyepiece  be  of  high  magnifying  power,  the  entire  sun  cannot 
be  seen  at  once.  Special  eyepieces  are  made  for  solar  work,  which 
obviate,  in  large  measure,  the  inconveniences  rising  from  its  intense 
light  and  heat. 

52.  The  Photosphere.  — The  .name  photosphere  (sphere  of  light) 
is  given  to  the  brilliant  surface  of  the  sun.  This  surface,  as  seen 
through  a  telescope,  has  a  grayish  cast.  It  is  brighter  at  the  centre 
than  near  the  edge.  The  cause  of  this  is  shown  in  Fig.  32.  To  an 


38  DESCRIPTIVE    ASTRONOMY. 

observer  situated  at  the  right  of  the  figure,  rays  coming  from  A  or 
B,  which  would  be  at  the  edge  of  the  sun,  pass  through  more  of  the 
sun's  atmosphere  than  those  coming  from  C ;  thus  they  suffer  a 
greater  absorption,  and  appear  fainter.  The  photosphere  corre- 
sponds to  the  crust  of  the  earth,  but  it  is  far  from  solid :  it  is  to  be 
regarded  as  a  cloud-like  shell  of  intensely  heated  vapors.  As  water 


Fig.  33. — FACUL^E  OBSERVED  VISUALLY. 

on  the  earth,  when  evaporated,  rises  and  condenses  in  the  upper  air 
into  clouds,  so  the  vapors  of  metals,  rising  from  the  interior  of  the 
sun,  condense  into  drops  and  form  the  photospheric  clouds. 

53.  Rice  Grains.  —  The  photosphere,  when  viewed  under  favorable 
conditions,  is  seen  to  be  mottled  with  small  bright  objects,  which 
have  been  likened  to  rice  grains  floating  in  a  plate  of  soup,  or  snow 


THE    SUN.  39 

flakes  on  a  gray  cloth.  To  an  observer  situated  a  few  thousand  miles 
from  the  earth,  the  cloud  forms  on  a  cloudy  day  might  exhibit  much 
the  same  appearance.  The  cloud  formation  which  navigators  call  a 
mackerel  sky  is  suggestive  of  it. 

54.  Faculae.  —  Faculae  (from  the  Latin  wordfacu/a,  a  small  torch) 
are  shown  in  Fig.  33.     They  are  best  seen  near  the  limb1  of  the  sun, 
and  are   especially  abundant  in  the   neighborhood  of  spots.     The 
photosphere  is  agitated  by  such  furious  storms  that  its  outer  surface 
rises  in  mountain-like  ridges,  or  crests,  like  the  waves   of  a  raging 
sea ;    these,  projecting  upward  through  the  lower  part  of  the  solar 
atmosphere,  look  brighter  than  the  general  background. 

A  man  standing  on  the  summit  of  Pike's  Peak  on  a  clear  night 
would  see  the  moon  through  much  less  of  our  light-absorbing  at- 
mosphere than  if  he  were  at  sea  level.  Hence  the  moon  would  appear 
more  glorious. 

In  like  manner,  an  observer  on  the  moon  could  see  high  terrestrial 
mountains  better  than  the  low  level  of  the  plains.  The  faculae  are 
more  distinct  near  the  limb  of  the  sun,  because  the  general  back- 
ground is  darker  there.  Recent  photographs,  however,  show  them 
well  near  the  centre  of  the  sun's  disk.  They  are  sometimes  20,000 
miles  long,  and  more  than  ,200  miles  high.  The  faculae  seem  to  form 
an  irregular  network  over  the  entire  surface  of  the  sun. 

SUN    SPOTS. 

55.  General  Appearance  of  a  Spot.  —  Sun  spots  reside  in  the  photo- 
sphere.    In  looking  at  the  sun  with  a  telescope,  specks   of  dust  on 
the  eyepiece  are  seen  as  black  spots  on  its  face.     But  a  true  sun  spot 
is  distinguished   from   these   by  the  fact  that  it  has  a  dark  central 
portion  surrounded  by  a  lighter  border.     The  dark  central  part  is 
called  the  umbra ;  the  border  is  the  penumbra.     Some  portions  of 
the  umbra  are  frequently  darker  than  others.     The  impression  given 
to   an  observer   is   that  these    dark   places   are   deep    holes.      The 
penumbra  is   composed  of  filaments  which  point  inward  toward  the 
umbra.     Sometimes  bridges  of  light  cross  the   umbra  from  side  to 
side,  or,  if  too  short,  seem  to  project  out  over  it,  as  a  fishing  rod 
hangs  over  a  pool. 

1  The  word  "limb"  is  used  by  astronomers  to  denote  the  edge  of  the  disk. 


4°  DESCRIPTIVE    ASTRONOMY. 

Occasionally,  when  a  spot  is  just  on  the  edge  of  the  sun,  it  is 
seen  as  a  notch  in  its  smooth  periphery.  Hence  spots  are  thought 
to  be  saucer-like  depressions  below  the  general  level. 


Fig.  34.  —  SUN  SPOT. 

(From  Langley's  "  New  Astronomy,"  by  permission.) 

.  56.  Changes  in  Appearance.  —  These  objects  are  not  of  fixed  form, 
like  mountains  or  lakes,  but  change  continually.  The  change  in  a 
day  is  usually  very  marked.  Bridges  may  form  or  disappear ;  the 
spot  may  grow  or  diminish  very  perceptibly,  may  break  into  two  or 
more  spots,  or  may  even  vanish  altogether.  The  changes  are  on 
some  occasions  so  rapid  as  to  make  it  impossible  to  sketch  the  spot. 
An  area  as  large  as  the  United  States  may  vanish  in  a  quarter  of  an 
hour.  The  lifetime  of  a  spot  averages  three  or  four  weeks ;  one  has 
been  known  to  last  a  year  and  a  half. 

57.  Dimensions.  —  The  smallest  ones  observed  with  large  telescopes 
have  umbrae  500  miles  in  diameter.  In  a  large  spot  the  diameter  of 
the  umbra  may  reach  50,000  miles.  The  largest  group  in  recent 


THE    SUN.  41 

years  was  visible  during  Feb.  5-17,  1892.  It  was  150,000  miles  long 
and  75,000  miles  broad,  the  central  spot  of  the  group  being  100,000 
miles  long,  and  half  as  broad.  Such  a  group  may  be  seen  without 
a  telescope,  by  looking  through  a  colored  or  smoked  glass,  or  even 
through  a  bank  of  haze  when  the  sun  is  near  the  horizon. 


Fig.  35.  —  LARGE  SUN  SPOT. 


58.  Movements :  Rotation  of  the  Sun.  —  If  a  spot  be  seen  at  noon  of 
a  certain  day  in  the  centre  of  the  sun's  disk,  at  the  next  noonday  it 
will  appear  at  the  right  of  its  former  position.  In  a  week  it  will 
have  passed  the  western  limb,  and  in  two  weeks  thereafter  it  will 
emerge  into  view  on  the  eastern  limb,  if  still  in  existence.  This 
shows  that  the  sun  rotates  on  an  axis,  as  the  earth  does.  Care- 
ful observations  made  on  spots  show  that  different  portions  of  the 
sun  rotate  in  different  times.  Near  the  solar  equator  spots  make 
their  circuit  in  twenty-five  days,  but  spots  situated  half  way  from 
the  equator  to  the  poles  consume  twenty-seven  days  in  a  like 
journey. 

Spots  are  rarely  seen  at  the  solar  equator,  and  never  more  than 
half  way  to  the  poles.  No  one  has  yet  explained  satisfactorily  this 


42  DESCRIPTIVE    ASTRONOMY. 

distribution  of  the  spots,  or  the  irregularity  of  the  rotation  time  of 
different  portions  of  the  sun's  surface. 

59.  Periodicity.  —  In  1826,  Schwabe,  a  magistrate  in  the  little 
German  town  of  Dessau,  began  for  his  own  pleasure  to  count  the 
number  of  sun  spots  visible  in  his  telescope  each  day.  After  twenty- 
five  years  of  patient  endeavor  he  found  that  he  had  been,  like  Saul, 


- 


•• 
Fig.  36.  —  THE  SUN  AT  THE  TIME  OF  A  SPOT  MAXIMUM. 

"  going  out  to  seek  his  father's  asses,  and  rinding  a  kingdom."  For 
he  discovered  that  spots  were  much  more  numerous  in  some  years 
than  in  others,  and  that  the  numbers  changed  with  considerable  regu- 
larity. Later  investigations  have  fixed  the  average  period  as  being 
1 1. 1  years.  A  maximum  of  spottedness  occurred  in  1893,  but  was 


THE    SUN. 


43 


not  very  pronounced.  After  that  the  spot  activity  gradually  lessened, 
and  is  expected  to  be  feeblest  about  1900.  Then  for  weeks  at  a  time 
no  spot  may  be  visible ;  after  the  minimum,  the  spotted  area  will 
increase  until  about  1905,  when  another  maximum  is  due;  at  a  time 
of  maximum  the  sun  is  never  free  from  spots.  Times  of  maxima 
and  minima  may  vary  a  year  or  two  from  those  predicted.  The 
cause  of  this  periodicity  is  unknown ;  it  has  been  surmised  to  be  due 
in  some  way  to  planetary  influences. 

60.    Observations  by  Carrington  and   Hodgson. — Very  violent  dis- 
turbances  are   at  times  noted  in  the  neighborhood  of  spots.     The 


Fig.  37-  —  PHOTOGRAPHS  OF  THE  DISTURBANCE  OF  JULY  15,  1892. 

classic  observation  of  Carrington  and  Hodgson,  two  English 
observers,  was  made  on  Sept.  I,  1859.  Near  the  edge  of  a  great 
spot  there  suddenly  appeared  two  luminous  masses,  the  length  of 
each  of  which  was  equal  to  the  earth's  diameter.  So  dazzling 
were  they  that  they  were  estimated  to  be  five  times  as  brilliant  as 
the  general  surface  of  the  sun.  They  moved  side  by  side  across  the 


44 


DESCRIPTIVE    ASTRONOMY. 


spot  with  a  velocity  of  over  100  miles  a  second,  growing  fainter;  in 
five  minutes  they  had  faded  from  view.  These  were  probably  the 
product  of  an  eruption  of  marvellous  energy. 

61.  Disturbance  on  July  15,  1892.  —  On  this  date  Prof.  George 
E.  Hale1  took  a  photograph  of  a  large  spot  which  had  two 
umbrae  separated  by  a  bright  bridge  of  light.  Another  photo- 
graph, taken  twelve  minutes  after,  showed  an  exceedingly  bright 
object,  shaped  somewhat  like  a  fish-hook,  the  hook  end  being 
baited  with  a  brilliant  ball  which  was  near  the  centre  of  the 
umbra.  In  half  an  hour  thereafter,  the  region  of  the  spot  was 
completely  covered  with  brilliant  outbursts,  so  that  the  umbrae 
were  no  longer  visible  in  the  photograph.  Two  hours  later, 
the  disturbance,  which  extended  over  an  area  of  four  billion 

square  miles,  had  disappeared 
entirely.  It  seems  to  have  been 
high  above  the  spots,  which  were 
unchanged  by  these  terrific  out- 
bursts. 

62.  Cyclonic  Motion.  —  On  rare 
occasions  a  spot  is  found  which 
exhibits  a  motion  of  rotation; 
sometimes  an  entire  revolution 
is  accomplished  in  a  few  days, 
but  usually  only  a  portion  of  a 
revolution  is  accomplished.  In 
such  spots  the  filaments  of  the 
penumbra  are  curved,  as  shown 
in  Fig.  38.  The  motion  of  these 
spots  is  analogous  to  that  of 

whirlwinds  and  cyclones  upon  the  earth.  But  the  analogy  must 
not  be  pressed  too  far,  for  terrestrial  cyclones  in  the  northern 
hemisphere  always  rotate  in  a  left-handed  direction  (opposite  to 
that  of  the  hands  of  a  watch).  Sun  spots  have  no  regularity  of 
rotation. 

63.    Nature  of  Sun  Spots.  —  Upon  this  there  has  been  much  specu- 
lation.     No   theory  has  yet    been   found  which   accounts   for  the 


Fig.  38.  —  CYCLONIC  MOTION  IN  A  SPOT. 


1  Director  of  the  Yerkes  Observatory. 


THE    SUN.  45 

observed  appearances  fully.  Prof.  Young's1  theory  is  given  in 
substance  below. 

When  the  fiery  gases  imprisoned  beneath  the  photospheric  cloud- 
shell  burst  forth  at  any  weak  place  in  the  shell,  there  is  a  temporary 
diminution  of  the  upward  pressure  against  the  photosphere  in  that 
locality.  Hence  the  photosphere  sinks  somewhere  in  the  neighbor- 
hood, an  irregular  shallow  cavity  being  formed.  The  materials 
thrown  out  by  the  eruption  are  cooled  in  the  upper  regions  of  the 
sun's  atmosphere,  and  fall  back  into  the  cavity.  The  light  from 
below,  struggling  up  through  this  mass  of  comparatively  cool  vapors, 
is  dimmed  by  absorption.  Hence  the  umbra,  though  really  intensely 
luminous,  sends  to  us  less  light  than  the  surrounding  photosphere, 
and  looks  black  by  contrast  with  it. 

The  filaments  of  the  penumbra  are  supposed  to  be  long  drawn 
out  rice  grains  (§  53). 

64.  Sun  Spots  as  Causes  of   Changes  of  the   Weather,  etc.  —  Many 
have  been  the  attempts  to  show  that  the  maxima  and   minima  of 
spots  affect  the  meteorological  conditions.     One  investigator  dis- 
covers that  years   when  sun  spots    are    at  a    maximum  are   more 
rainy  than  the  average,  and  that  cyclones  and  other  violent  storms 
are  then  most  prevalent.     Another  concludes  that  such  years  are 
hotter  than   the  average,   while  a  third    finds    them   to   be   cooler. 
Others  attribute  the  recurrence  of  Asiatic  cholera,  variations  in  the 
amount  of  atmospheric  ozone,  or  the  prevalence  of  commercial  panics, 
to  the  direful  spots.    The  data  on  which  these  conclusions  are  based 
are,  in  general,  so  conflicting  as  to  produce,  in  one  who  examines 
them,  much  weariness  of  the  flesh  and  little  satisfaction  of  the  spirit. 

65.  Magnetic  Storms.  — A  compass  needle  does  not  always  point 
in  the  same  direction.     One  of  the  large  and  accurate  ones  used  in 
magnetic    observatories   shifts    in    direction    a    few  minutes    of  arc 
every  day,  vibrating  to  and  fro.     Sometimes  these  oscillations  are 
greatly  increased,  and  are  subject  to  no  perceptible  law ;   the  needles 
seem  fairly  beside  themselves  with  magnetic  excitement. 

Powerful  currents  traverse  the  telegraph  wires,  and  send  mes- 
sages in  an  unknown  tongue ;  private  lines  are  temporarily  worked 
in  the  nervous  systems  of  the  operators;  the  regular  electrical 

1  C.  A.  Young,  Professor  of  Astronomy  in  Princeton  University,  one  of  the  most 
distinguished  students  of  the  sun. 


46  DESCRIPTIVE    ASTRONOMY. 

apparatus  is  set  on  fire  at  times.     At  night  the  weird  auroral  beams 
execute  their  most  fantastic  dances. 

66.  Connection  of  these  Storms  with  Solar  Outbursts.  —  The  singular 
event  mentioned  in  §  60  took  place  during  a  great  magnetic  storm, 
which  was  raging  upon  the  earth.     In  Washington  and  Philadelphia 
the  telegraph  operators  were  severely  shocked.     At  Boston  a  flame 
of  fire  followed  the  pen  of  a  recording  telegraphic  instrument. 

There  were  fine  auroral  displays  in  all  parts  of  the  world ;  even 
countries  near  the  equator  enjoyed  the  spectacle,  to  them  almost 
unknown.  During  the  years  1873  to  1892  there  were  three  especially 
severe  magnetic  storms  on  the  earth.  There  were  also  three  very 
notable  displays  of  sun  spots.  The  magnetic  storms  occurred  at 
the  times  of  the  greatest  development  of  the  spots. 

67.  The  Storm  of  February,  1892.     The  great  spot  of  Feb.  5-17, 
1892    (§  57),  was  accompanied  by  a  magnetic  storm  which  raged 
on    Feb.   13   and    14,   when  the  spot  group  had    attained  its  max- 
imum   dimensions,  covering  -3^    of  the    sun's    visible  hemisphere. 
Fine  auroras  flashed  out  during  this  storm.     Magnetic    recording 
instruments  were  more  violently  disturbed  than  for  ten  years  pre- 
viously.    An  earth  current  awakened  a  sleeping  operator,  in  France, 
by  ringing   his  signal   bell.     Nearly  a  month  afterwards,  when  the 
spot,  much  enfeebled,  came  by  reason  of  the  sun's  rotation  into  the 
same  apparent  position  on  its  disk,  another  bright  aurora  accom- 
panied  by  a  magnetic   storm  occurred. 

68.  Frequency    of    Magnetic    Storms.  —  An    examination    of    the 
records  of  these  storms  shows  that  they  too  have  times   of  maxi- 
mum and  minimum,  and  that  these  times  correspond  closely  with 
those  for  sun   spots.     That  there   is  some   connection  between  the 
two  is  no  longer  doubtful,  though  the  most  distinguished  physicists 
are  unable  to  explain  the  nature  of  the  relation.     Conspicuous  sun 
spots,  or  other  solar  disturbances,  are   not  always  accompanied   by 
magnetic  storms  on   the  earth.     This  is  not    astonishing,  however; 
for  terrestrial  storms  often  occur  in  which  there  is  no  special  dis- 
play of  electrical  phenomena. 

Some  are  of  the  opinion  that,  when  a  solar  storm  is  associated 
with  electrical  disturbance  there,  the  disturbance  is  propagated  with 
the  speed  of  light  through  the  ether  to  the  earth,  which  is  thrilled 
responsively. 


THE    SUN. 


47 


THE    SPECTROSCOPE. 


69.    Description   of  the   Instrument.  —  We   learned  in    §   37,  that 
white  light  might  be  resolved  into   its  component  colors  by  pass- 


Fig.  39-  — A  SPECTROSCOPE  (MADE  BY  BRASHEAR). 

ing   it   through  a  prism.     The  peculiarities  of  light  thus  dispersed 
are  conveniently  studied  by  means  of  the  spectroscope ;    the  action 


Fig.  40.  —  PLAN  OF  A  SPECTROSCOPE. 

of  a  simple  form  of  this   instrument  is  shown  by  Fig.  40.     At  the 
point  S  is  a  slit,  shown  in  Fig.  41.     It  is  a  straight  narrow  opening 


48  DESCRIPTIVE    ASTRONOMY. 

between  two  pieces  of  metal,  x  and  j,  shown  in  the  cut ;  x  is  movable 
by  the  screw  a,  so  that  the  width  of  the  slit  may 
be  altered  at  pleasure.  S  is  put  at  such  a  dis- 
tance from  the  lens  A  that  the  rays  of  light 
coming  from  S  are  rendered  parallel  by  pass- 
ing through  A.  These  rays  then  strike  the 

Fig.  41.  — SLIT  OF  A  SPEC-         .  r  i    t      «A  ,1,1 

TROSCOPE.  prism,  are  refracted  by  it,  enter  the  telescope, 

and  come  through  to  the  eye  at  E. 

70.  Slit  Illuminated  by  Red  Light.  —  Suppose  that  in  front  of  the 
slit  we  could  burn  some  substance  which   gave    out  a  red  light,  no 
other  color  except  a  particular  shade  of  red  being  given  out  by  the 
substance.     Let  the  slit  be  almost  closed.     On  looking  through  the 
telescope  one  would  see  a  fine  red  line,  just  the  shape  of  the   slit. 
If  half  the  slit  were  covered  by  a  card,  the  observer  would   see  a 
line  only  half  as  long  as  before ;   if  the  slit  were  widened  by  turning 
the  screw  a  (Fig.  41),  the  image  seen  by  the  observer  would  be 
widened  likewise.     If  a  small  circular  hole  were  put  in  place  of  the 
slit,  the  observer  would  see  through  the  telescope  a  red  circle. 

71.  Slit  Illuminated  by  Lights  of  Different  Colors.  —  The  flame  of 
an    alcohol    lamp    is    almost   colorless.     Place    on   the  wick    some 
common  salt  and  the  flame  will  be  colored  yellow,  this  hue  being 
due  to  sodium.     Put  the  yellow  flame  in  front  of  the  slit,  and  the 
observer  will  see  a  yellow  line,  the  image  of  the  slit.     Try  a  similar 
experiment  with  a  salt  of  thallium,  and  a  green  slit  image  will  be 
seen.     Next  lay  on  the  wick  of  the  lamp  both  common  salt  and  a 
salt  of  thallium.     Both  yellow  and  green  light  will  enter  the  slit,  but 
in  passing  through  the  prism  the  yellow  rays  will  not  be  bent  out  of 
their  course  as  much  as  the  green  rays.     Therefore,  if  the   slit  be 
nearly  closed,  the  observer  will  see  two  fine  lines,  one  yellow  and 
the  other  green,  standing  side  by  side.     If  any  number  of  colors  be 
admitted  at  once,  there  will    be   tlie   same  number  of  slit  images 
standing  side  by  side.     When  a   candle,  «which   gives  a  light  com- 
posed of  a  great  number  of  t'ints,  illuminates   the  sli%  the   images 
are  so  closely  crowded  together  that  they  form  a  continuous  band 
of  color  from  red  to  violet  (§  37).     This  is  called  a  continuous  spec- 
trum.    Were  the  candle  capable  of  giving  out  all  colors  but  green, 
there  would  be  in  the  ribbon  of  light  or  spectrum  a  dark  gap  be- 
tween cyan-blue  and  yellow. 


THE    SUN. 


49 


72.  White  Light  shining  through  an  Incandescent  Gas.  —  Arrange 
the  apparatus  as  shown  in  Fig.  42.  Let  the  sodium  and  thal- 
lium be  giving  in  the  spectroscope  their  yellow  and  green  lines. 
Lift  up  the  screen  so  that  the  calcium  light1  shines  through  the 
glowing  gases  in  the  flame  of  the  spirit  lamp  into  the  instrument. 


zt 


is 

C/3 


LAMP 
Fig.  42.  —  PRODUCTION  OF  SPECTRA. 


Instantly,  the  spectrum  will  change  to  a  many-colored  ribbon,  like 
that  caused  by  a  candle,  except  that,  where  the  bright  lines  due  to 
sodium  and  thallium  formerly  were,  the  spectrum  will  be  crossed 
by  dark  lines.  Such  a  spectrum  is  called  an  absorption  spectrum. 
The  two  spectra  are  shown  without  the  colors  in  Fig.  43.  Put  the 


GREEN 


YELLOW 


BRIGHT  LINE 
SPECTRA 


THALLIUM 
LINE 

50CHUM 
LINES 

DARK  LINE 
5PECTRA 

Fig.  43. — SPECTRA. 

•  ; 

screen  in  place  again,  so  as  to*  cut' off  the  rays  from  the  calcium 
light,  and  the  bright  lines  will^eappear.  Remove  the  alcohol  lamp 
and  the  screen,  and  the  calcium  light  will  produce  a  continuous 
spectrum. 

1  The  calcium  light  is  produced  by  introducing  a  piece  of  lime  into  a  flame  caused 
by  burning  oxygen  and  hydrogen  gases  together.  Such  a  light  has  been  seen  over  one 
hundred  miles  in  full  daylight. 

4 


DESCRIPTIVE    ASTRONOMY. 


73.  Laws  of  Spectrum  Analysis.  —  By  an  exhaustive  series  of  experi- 
ments similar  to  the  preceding,  the  following  laws  have  been 
discovered.  They  are  called  Kirchhoff's  laws. 

I.  An  incandescent  solid,  or  liquid,  or  even  gas  under  high 
pressure,  gives  a  continuous  spectrum. 

A  candle  or  kerosene  lamp  gives  a  continuous  spectrum  because 
nearly  all  of  its  light  comes  from  glowing  particles  of  solid  carbon 

(which  when  cooled  form  soot). 

2.  A  glowing  gas,  unless  con- 
densed by  high  pressure,  gives  a 
discontinuous  spectrum  made  up  of 
bright  lines  or  bands. 

The  spectrum  of  iron  vapor  con- 
sists of  hundreds  of  bright  lines. 
Sodium  vapor  gives  a  small  number 
of  lines,  the  most  conspicuous  of 
which  has  been  mentioned ;  with 
a  spectroscope  of  high  dispersive 
power,  such  as  one  in  which  the  light 
is  passed  through  several  prisms, 
this  line  is  seen  double.  The  spec- 
trum of  a  gas  is  usually  obtained 
by  passing  electrical  discharges 
through  a  Geissler's  tube  con- 
taining the  gas.  The  narrow  por- 
tion of  the  tube  has  a  very  small  bore,  and  the  gas  in  it  glows 
brightly. 

3.  A  gas  absorbs  from  white  light  passing  through  it  those  rays 
which  the  gas  itself  when  incandescent  emits.  This  law  explains  ab- 
sorption spectra.  When  sodium  vapor  in  the  flame  of  the  spirit  lamp 


Fig.  44.  —  KlRCHHOFF,  THE  DISCOVERER  OF 

THE  LAWS  OF  SPECTRUM  ANALYSIS. 


Fig.  45.  —  A  GEISSLER'S  TUBE. 

is  interposed  between  the  slit  of  the  spectroscope  and  the  calcium 
light,  it  absorbs  much  of  the  yellow  light  coming  from  the  lamp 
which  would  otherwise  have  fallen  upon  that  place  in  the  spectrum 
where  the  main  sodium  line  is  located.  This  place  in  the  spectrum 


THE    SUN.  51 

is  therefore  lighted  up  only  by  the  light  coming  from  the  sodium 
vapor  and  a  portion  of  the  yellow  light  coming  from  the  lime. 
Hence  it  looks  dark  by  contrast  with  the  rest  of  the  spectrum  which 
is  brightly  illuminated  by  the  lime  light. 

It  is  found  that  the  heated  vapor  of  any  elementary  substance,1 
(like  sodium,  iron,  aluminum,  oxygen,  etc.,)  when  at  a  given  tem- 
perature and  pressure,  has  for  its  spectrum  a  particular  group  of 
bright  lines  by  which  it  is  distinguished  from  other  elements. 


rni 


.Fig.  46.  —  A  PORTION  OF  THE  SOLAR  SPECTRUM. 

74.  The  Solar  Spectrum.  —  When  sunlight  is  admitted  through/the 
slit  of  a  spectroscope,  it  gives  an  absorption  spectrum,  the  lines 
being  very  numerous ;  a  number  of  them  are  shown  in  Fig.  46. 


IIIIIIH 


Illlllllllll 

I  L  J2:L1 


Fig.  47.—  CORRESPONDENCE  OF  BRIGHT  AND  DARK  LINES 
IN  Two  SPECTRA. 

From  the  third  law  we  conclude  that  the  light  coming  from  the 
photosphere  has  passed  through  some  gas  or  gases  on  its  way  to  us. 
In  order  to  find  out  what  these  gases  are,  we  compare  the  solar 
spectrum  with  the  bright  line  spectra  of  various  gases.  This  is 
done  by  admitting  sunlight  through  one  half  of  the  slit,  while  the 

1  For  a  list  of  the  elements,  consult  any  work  on  chemistry. 


52  DESCRIPTIVE    ASTRONOMY. 

light  from  some  glowing  gas  is  admitted  through  the  other  half. 
One  who  looks  through  the  telescope  sees  one  spectrum  above  the 
other,  as  shown  in  Fig.  47,  which  exhibits  the  correspondence 
between  bright  and  dark  lines  in  portions  of  two  spectra. 

We  conclude  that  whenever  the  bright  lines  in  the  spectrum  of 
some  particular  glowing  gas  correspond  to  certain  dark  lines  in  the 
solar  spectrum,  that  gas  is  present  in  the  atmosphere  of  the  sun. 

75.  Constituents  of  the  Sun :  Telluric  Lines.  —  By  comparison  of  the 
spectra  of  various  vapors  with  that  of  the  sun,  it  has  been  shown 
that  many  of  the  substances  found  on  the   earth  exist  in  the   sun 
also.     Some  of  the  most  commonly  known  of  these  are  iron,  carbon, 
hydrogen,  nickel,  and  copper.     No  trace  has  been  found  of  such 
important  elements   as   chlorine,  nitrogen,  and   mercury.     But   the 
spectra  of  these,  when  heated  to  an  enormous  temperature,  as  at  the 
sun's   surface,   may  be  very  different  from  those  produced  in  our 
laboratories. 

Lockyer :  has  advanced  the  theory  that  substances  regarded  as 
elements  are  really  compounds  which  are  separated  into  their  con- 
stituents by  the  intense  heat  at  the  sun,  so  that  their  spectra  are 
much  changed. 

Many  of  the  dark  lines  in  the  solar  spectrum  are  caused  by 
absorption  in  passing  through  our  atmosphere.  These  are  called 
telluric  (tellus,  the  earth)  lines. 

THE    SUN'S    SURROUNDINGS. 

76.  The  Chromosphere.  —  The  chromosphere  is  that  portion  of  the 
sun's    u  atmosphere "    which    lies  next    to  the  photosphere.     It   is 
visible  during  a  total  solar  eclipse,  when  the  moon  has  hidden  the 
photosphere   from  view;    its  color   is   scarlet.     It   is   composed   of 
upright  filaments,  and  has  the  appearance  of  a  stubble-field,  the 
"  stubble  "  averaging  over  five  thousand  miles  in  height.    Hydrogen, 
helium,  and  calcium  are  its  principal  constituents. 

Helium  received  its  name  from  the  Greek  word  helios  (the  sun), 
because  it  was  supposed  to  exist  in  the  sun  only.  But  when  Dr. 
Ramsey2  was  examining  a  specimen  of  a  species  of  pitchblende  in 

1  J.  Norman  Lockyer,  an  English  astronomer,  who  is  among  the  foremost  of  living 
spectroscopists. 

2  An  English  physicist,  one  of  the  discoverers  of  argon. 


THE    SUN. 


SOLAR    PROMINENCES. 


THE    SUN.  53 

1895,  he  detected  helium  in  it.  It  has  since  been  found  in  certain 
mineral  springs  in  Europe,  and  in  several  rare  minerals,  though 
always  in  small  quantities.  It  is  now  known  to  be  widely  distributed 
throughout  the  universe,  for  lines  due  to  it  are  in  the  spectra  of 
stars  and  nebulae. 

77.  Prominences  or  Protuberances.  —  At  the  time  of  a  solar  eclipse 
many  fantastic  crimson  objects  are  seen  jutting  out  from  the  chromo- 
sphere at  the  sun's  limb.     They  are  divided  into   two   classes,  the 
cloudlike  or  quiescent,  and  the  eruptive. 

They  are  shown  in  Figs.  I  and  48. 

The  former  are  immense  irregular  masses  which  overhang  the 
chromosphere,  looking  like  the  thunder-heads  which  lazily  bask  in 
the  sunshine  on  a  quiet  summer  afternoon.  Usually  they  are 
connected  with  the  chromosphere  by  columns  which  remind  one  of 
pictures  of  terrestrial  water-spouts.  Sometimes  they  last  a  month. 
One  has  been  seen  which  was  475,000  miles  high:  its  extreme 
apparent  breadth  was  about  the  same.  They  are  of  the  same  com- 
position as  the  chromosphere. 

Eruptive  prominences  are  fiery  fountains  of  gas  which  spurt 
out  from  the  chromosphere.  Some  fine  specimens  are  shown  in 
Fig.  I.  One  has  been  known  to  rise  to  a  height  of  350,000  miles. 
In  these  prominences  not  only  chromospheric  matter,  but  some  of 
the  vapors  of  the  photosphere  are  carried  up,  with  velocities  which 
baffle  comprehension.  On  May  5,  1892,  a  velocity  of  323  miles 
per  second  was  measured.  This  eruption  probably  hurled  masses 
of  glowing  gas  entirely  away  from  the  sun. 

78.  Prominences  seen  with  the  Spectroscope.  —  Prominences  are  not 
visible  with  a  simple  telescope  except  at  the  time  of  an  eclipse.     But 
if  a  spectroscope  of  good  dispersive  power  be  attached  at  the  eye- 
end  and  properly  adjusted,  one  may  study  the  prominences  on  any 
clear  day.     (See  Fig,  49.)     The  explanation  of  this  may  be  found 
in  large  works  on  astronomy.1 

Prominences  are  distributed  all  over  the  sun,  but  are  seen  only 
at  its  limb.  Those  upon  its  face  are  invisible  because  the  photo- 
sphere back  of  them  is  so  bright. 

79.  Prominences   and   Magnetic   Storms.  —  Prominences,    like    sun 
spots,  are  periodic ;  their  times  of  maximum  and  minimum  coincid- 

1  See  Young's  General  Astronomy,  Art.  324. 


54 


DESCRIPTIVE    ASTRONOMY. 


ing  with  those  of  the  spots.     Like  the  spots,  they  are  associated  with 
magnetic  storms. 


Fig.  49. — A  SPECTROSCOPE  ATTACHED  TO  A  TELESCOPE. 

Prof.  Young,  when  observing  at  a  mountain  station  during  the 
forenoon  of  August  3,  1872,  noticed  especial  activity  of  prominences, 
jets  of  unusual  brightness  being  ejected.  At  dinner  time  one  of  the 
party,  who  had  been  taking  magnetic  observations,  and  who  did  not 
know  what  Prof.  Young  had  seen,  said  that  he  had  been  obliged  to 


THE    SUN. 


55 


desist,  because  the  magnet  had  swung  clear  off  the  scale.  Three 
times  during  the  forenoon  especially  violent  disturbances  were 
observed,  and  at  those  times  the  magnetic  needles  in  English  obser- 
vatories exhibited  great  fluctuations. 

80.  Appearance  of  the  Corona.  —  At  the  moment  when  the  last 
ray  of  sunlight  vanishes,  in  a  solar  eclipse,  there  bursts  upon  the 
vision  a  pearly  radiance  of  wonderful  beauty,  which  is  shown  in 
Fig.  50.  This  is  the  corona,  so  called  because  it  is  a  crown  upon 
the  King  of  Day. 


Fig.  50.  —  THE  CORONA  ON  JULY  29,  1878. 

Near  the  sun  it  is  almost  dazzling  in  brightness,  but  it  fades  away 
into  faint  streamers  and  tufts  of  light  which  sometimes  extend  to 
great  distances.  Observers  on  the  summit  of  Pike's  Peak,  in  1878, 
saw  streamers  9,000000  miles  in  length.  In  the  telescope  the  inner 
bright  part  of  the  corona  is  seen  to  be  composed  of  innumerable 
fine  filaments,  like  the  dishevelled  blonde  tresses  of  some  mountain 
nymph.  Fig.  5 1  is  from  a  photograph  of  the  corona.  The  corona 
differs  widely  in  appearance  at  different  eclipses. 

During  1895,  Mr.  D.  E.  Packer1  made  the  capital  discovery  that 

1  Of  South  Birmingham,  England. 


56  DESCRIPTIVE    ASTRONOMY. 

the  corona  could  be  photographed  on  any  clear  day  through  a  thin 
metallic  screen.  Coronal  light,  like  the  Rontgen  rays,  penetrates 
such  screens.  The  photographs  show  that  there  is  an  intimate  con- 
nection between  the  corona  and  active  sun  spots,  as  every  promi- 


Fig.  51.  —  THE  CORONA,  PHOTOGRAPHED  ON  DEC.  21,  1889. 

nent  filament  points  toward  some  spot.     Many  of  the  filaments  are 
twisted,  like  a  corkscrew.1 

81.  Schaeberie's  Theory  of  the  Corona.  —  Prof.  Schaeberle2  has  ad- 
vanced a  theory  that  the  corona  is  caused  by  the  ejection  of  numer- 
ous streams  of  matter,  driven  by  forces  which  are  most  active  near 
the  centre  of  the  zones  in  which  spots  are  found.  Owing  to  the 
sun's  rotation  these  streams  are  curvilinear,  and  appear  to  interlace. 

J  Mr.  Packer's  work  still  (July,  1896)  awaits  confirmation  by  other  astronomers,  and 
may  prove  to  be  illusory. 

2  J.  M.  Schaeberle,  Astronomer  at  the  Lick  Observatory,  Mt.  Hamilton,  Cal. 


THE    SUN.  57 

The  variations  in  the  form  of  the  corona  at  various  eclipses  are 
partially  explained  by  the  fact  that  our  point  of  view  is  continually 
changing,  as  the  earth  pursues  its  annual  journey  about  the  sun. 


Fig,  52.  —  ILLUSTRATIONS  OF  SCHAEBERLE'S  THEORY  OF  THE  CORONA. 

Prof.  Schaeberle  took  a  ball  to  represent  the  sun,  and  thrust  into 
it  a  large  number  of  needles,  in  the  regions  corresponding  to  those 
where  spots  on  the  sun  are  most  numerous.  The  ball  was  then 
placed  in  several  positions,  and  photographed  :  two  of  the  results 
are  shown  in  Fig.  52. 


Fig.  53«.  —  A  DRAWING  OF  THE  CORONA. 

The  photographs  of  the  eclipse  of  April  16,  1893,  are  thought 
by  Prof.  Schaeberle  to  confirm  his  theory. 

82.  Nature  of  The  Corona.  —  The  corona  gives  two  spectra,  one 
made  up  of  bright  lines,  the  other  continuous.  The  bright  line 
spectrum  comes  from  a  glowing  gas.  The  continuous  spectrum  may 
come  from  incandescent  solid  or  liquid  particles  scattered  through 


50  DESCRIPTIVE    ASTRONOMY. 

the  corona,  or  from  the  light  of  the  photosphere  reflected  from  the 
materials  of  the  corona. 


Fi§-  53^-  —  A  DRAWING  OF  THE  CORONA. 

The  bright  line  spectrum,  when  carefully  examined,  reveals  the 
presence  of  an  unknown  element,  which  has  been  called  coronium ; 
hydrogen  is  also  found.  No  complete  explanation  of  the  phenomena 


—  A  DRAWING  OF  THE  CORONA. 


Fig.  53^.  —  A  PHOTO- 
GRAPH OF  THE  IN- 
NER CORONA. 


exhibited  by  the  corona  has  yet  been  found,  but  it  is  not  improbable 
that  electrical  action  may  account  for  many  of  them.  Fig.  54  shows 
some  effects  produced  by  Dr.  Pupin,1  by  electrical  discharges  around 


M.  I.  Pupin,  Columbia  College,  New  York. 


THE    SUN. 


59 


a  brass  sphere  placed  in  a  globe  from  which  the  air  was  largely  ex- 
hausted. They  are  strikingly  like  coronal  forms.  Similar  experi- 
ments have  been  made  in  Germany  by  Dr.  Ebert  and  Prof.  Wiede- 
mann.  These  experiments  show  the  raylike  structure  and  silvery 
light  of  the  corona,  the  dark  rifts  which  are  frequently  seen  extend- 
ing from  the  sun's  limb  to  the  limit  of  the  corona,  and  the  abnormally 
long  streamers  which  have  graced  the  sun  during  certain  eclipses. 


Fig.  54.  —  ELECTRICAL  APPEARANCES  SIMILAR  TO  THE  CORONA. 

83.  Light  of  the  Sun.  —  Under  the  clear  sky  of  Colorado,  a  news- 
paper may  be  read  by  any  person  of  normal  eyesight  by  the  light 
of  the  full  moon.  How  100,000  full  moons,  crowding  the  vault  of 
the  heavens  would  blind  us  by  their  radiance  !  Yet  the  sun  gives 
us  six  times  as  much  light.  If  an  electric  arc  light  be  placed  be- 
tween the  eye  and  the  sun,  and  both  be  viewed  through  a  dark 
glass,  the  arc  light  will  appear  as  a  dark  spot  on  the  face  of  the  sun. 
Since  the  earth  receives  only  2.200,000000  °f  tne  light:  radiated  by 


6O  DESCRIPTIVE    ASTRONOMY. 

the  sun,  the  amount  of  light  radiated  in  all  directions  by  the  sun  is 
2,200,000000  times  as  great  as  that  which  the  earth  receives.  (See 
exercise  3,  at  the  end  of  this  chapter.)  This  inconceivable  quantity 
of  light  is  shot  through  space  with  a  velocity  of  186,330  miles  per 
second.  Were  the  sun  divested  of  its  atmosphere,  it  would  probably 
be  three  times  as  bright,  and  blue  in  color.  The  blue  rays  are  now 
strongly  absorbed  by  its  atmosphere. 

84.  Heat  of  the  Sun.  —  By  letting    the     sun    shine    for  a  given 
length  of  time   upon   the  blackened  cover  of  a  box  filled  with   a 
known  quantity  of  water,  and  by  noting  the  rise  of  temperature  in 
the  water,  it  is  possible  to  find  approximately  the  amount  of  heat 
received  by  the  earth  from  the   sun.     Calculation  based  on  such 
measurements  has  shown  that  the  sun  sends  to  the  earth    every 
second   enough  heat  to  raise  600,000000  tons  of  ice  water  to  the 
boiling  point. 

Imagine  that  a  gigantic  fire-engine  was  throwing  at  the  sun  a 
stream  of  water  75,000  miles  in  cross  section,  at  the  rate  of  1,000 
miles  a  second.  The  water  would  be  turned  into  steam  as  fast  as  it 
advanced,  if  the  entire  heat  of  the  sun  were  concentrated  upon  it. 
Stationary  engines  have  been  run  by  concentrating  the  sun's  heat 
by  means  of  huge  reflectors.  In  case  some  economical  way  of 
storing  and  distributing  heat  energy  were  discovered,  solar  engines 
might  take  the  place  of  coal-burning  ones. 

Langley l  says  that,  "  even  on  such  a  little  area  as  the  island  of 
Manhattan,  or  that  occupied  by  the  city  of  London,  the  noontide 
heat  is  enough,  could  it  all  be  utilized,  to  drive  all  the  steam  engines 
in  the  world." 

85.  Causes  of  the  Sun's  Radiation :   Combustion :  Meteoric  Theory.  — 
It  is  certain  that  the  outpour  of  heat  and  light  is  not  kept  up  by 
mere  combustion.     Had  the  sun  been  a  solid    mass    of  the    best 
anthracite,  burning  swiftly  enough  to  produce  the  known  supply  of 
heat,  less  than  6,000  years  would  have  been  required  for  its  complete 
consumption. 

There  has  been  a  theory  that  a  continual  rain  of  small  bodies 
falling  upon  the  sun  from  adjacent  space  keeps  up  the  supply  of 
heat.  We  see  evidences  of  such  production  of  heat,  when  a  cannon 

1  Dr.  S.  P.  Langley,  Secretary  of  the  Smithsonian  Institution,  Washington,  D.  C. 


THE    SUN. 


6l 


ball  strikes  an  armor  plate,  and  both  are  heated  by  the  impact 
This  is  known  as  the  "  meteoric  theory."  While  the  sun  doubtless 
receives  some  of  its  heat  from  such  a  pelting,  the  most  careful  inves- 
tigations show  that  only  a  minute  fraction  of  its  heat  can  come  from 
such  a  source.  For  if  the  sun  be  thus  bombarded,  why  not  the 
earth,  though  to  a  much  less  degree,  on  account  of  its  smaller  size 
and  feebler  attraction? 

Calculation  has  shown  that,  upon  this  theory,  each  square 
mile  of  the  earth  would  be  bombarded  by  fifty  tons  of  missiles 
every  day. 

86.  The  Contraction  Theory.  —  If  a  body  be  dropped  from  the  top 
of  a  high  tower,  heat  will  be  produced  when  its  motion  is  arrested 


Fig.  55.  — PRODUCTION  OF  HEAT  BY  THE  ACTION  OF  GRAVITY. 

by  striking  the  earth.  If  it  be  made  to  fall  slowly,  by  being  used  as 
a  weight  to  drive  a  machine,  as  in  Fig.  55,  heat  will  still  be  pro- 
duced. In  the  machine  shown  in  the  cut,  the  revolution  of  the 
paddles  heats  the  water.  Now,  we  conceive  that  the  entire  mass  of 
the  sun  is  shrinking  slowly,  each  particle  (except  the  one  at  the 
centre)  gradually  falling  inward.  This  process  will  generate  heat. 
So  enormous  is  the  sun's  mass  that  the  rate  of  contraction  necessary 
to  keep  up  the  supply  of  heat  is  very  slow,  being  only  ten  inches  a 
day.  The  amount  of  contraction  during  the  past  6,000  years  would 
not  be  noticeable,  even  with  the  best  modern  telescope.  This  theory 
is  generally  accepted  by  astronomers  as  the  best  which  has  been 
advanced. 


62  DESCRIPTIVE    ASTRONOMY. 

87.  Past  and  Future  of  the  Sun.  —  If  the  sun  has  been  radiating 
heat  uniformly  in  all  directions,  at  the  same  rate  as  now,  during  its 
entire  past,  and  if  the  heat  has  been  kept  up  by  contraction  alone, 
however  large  it  may  originally  have  been,  in  less  than  i8,oocpoo 
years  it  would  have  shrunk  to  its  present  dimensions.    Since  the  time 
when  its  diameter  was  equal  to  that  of  the  orbit  of  Mercury,  it  has 
radiated  over  eighty  times  as  much  heat  as  previously,  according  to 
this  theory.      By  the  use  of  similar  assumptions  it  has  been  guessed 
that  the  sun  will  not  give  enough  light  and  heat  to  supply  the  needs 
of  man  for  more  than  io,ooqooo  years  hence.     These  figures  might 
be  awe-inspiring,  if  the  foundations  on  which  they  rest  were  more 
substantial  than  "  the  baseless  fabric  of  a  vision."    For  it  is  extremely 
improbable  that  we  can  reason  with  any  approach  to  exactness  from 
the  slender  data  at  our  disposal.     The  sun  may  be  much  older  or 
younger.     As  Sir  William  Thomson 1  once  said,  in  a  lecture  on  this 
subject,  "  After  all,  we  don  't  know  anything  about  it." 

88.  The  Sun's  Constitution.  —  The  interior  of  the  sun  is  supposed 
to  be  gaseous  on   account  of  the    intense   heat,  the    gases    being 
extremely  compressed  by  the  weight  of  the  huge  solar  bulk. 

Surrounding  this  interior  is  the  photosphere,  a  cloud  shell  formed 
of  vapors  which,  though  they  have  been  condensed  by  exposure  to 
the  cold  of  surrounding  space,  are  yet  very  hot. 

The  chromosphere  comes  next,  composed  of  gases  not  so  easily 
condensed  as  the  materials  of  the  photosphere;  chief  among  these 
is  hydrogen. 

Mingled  with  the  chromosphere,  but  extending  to  vastly  higher 
elevations,  is  the  mysterious  corona,  made  up  of  rare  gases,  through 
which  are  scattered  finely  divided  particles  of  matter,  which  might 
remind  us  (if  we  could  see  them)  of  motes  floating  in  a  sunbeam. 

EXERCISES. 

89.  I.  If  light  travelling  186,330  miles  per  second  consumes  8m.. 
19  sec.  in  coming  from  the  sun  to  us,  find  the  sun's  distance  from 
the  earth. 

2.  In  §  83  it  is  estimated  that  100,000  full  moons  would  more 
than  fill  the  visible  hemisphere  of  the  sky.  Let  us  find  out  how  the 

1  Now  Lord  Kelvin,  the  great  mathematical  physicist. 


THE    SUN.  63 

number  was  computed.  The  moon  is  240,000  miles  from  us,  and 
its  radius  is  1,080  miles.  Imagine  that  the  visible  sky  is  a  hemi- 
spherical surface,  the  radius  of  which  is  240,000  miles,  and  that  the 
moon  is  a  circle,  the  radius  of  which  is  1,080  miles,  located  on  the 
hemispherical  surface.  The  area  of  a  hemisphere  =  2  X  3.1416  X 
the  square  of  its  radius.  The  area  of  a  circle  =  3.1416  X  the  square 
of  its  radius.  How  many  times  the  area  of  the  circle  is  the  area  of 
the  hemisphere? 

3.  In  §  83  it  is  stated  that  the  earth  receives  only  2.200.000000 
of  the  light  and  heat  sent  out  by  the  sun.     Imagine  a  huge  soap- 
bubble,  the  centre  of  which  is  at  the  sun,  its  radius  being  the  distance 
from  the  sun  to  the  earth,  93,000000  miles.     Change  the  film  to  a 
thin  crystal  shell  in  which  an  emerald  8,000  miles  in  diameter  is  set, 
to  represent  the  earth.     Remove  the   emerald,   leaving  a  circular 
hole  8,000  miles  across.     The  light  which  strikes  the  crystal  sphere 
in  one  second  equals  the  total  light  emitted   by  the  sun    in    one 
second.     The  light  which  streams  through  the  hole  in  one  second 
equals  the  amount  received  by  the   earth  in   one   second.     Hence> 
as  the  area  of  the  hole  is  to  the  area  of  the  sphere,  so  is  the  amount 
of  light  the  earth  receives  in  one  second  to  the  total  light  given  out 
by  the  sun  in  one  second. 

The  area  of  the  surface  of  a  sphere  =  4  X  3.1416  X  the  square  of 
its  radius.  The  area  of  a  circle  =  3.1416  X  the  square  of  its  radius. 
Compute  the  fractional  part  of  the  sun's  radiation  which  strikes  the 
earth. 

4.  Why  cannot  the  prominences  and  corona  be-  seen  with  a  good 
telescope  on  any  bright  day? 

5.  What  change  does  the  vapor  which  is  shot  off  from  the  sun 
(§  77)  undergo,  in  passing  through  space?" 


\ 


64  DESCRIPTIVE    ASTRONOMY. 


CHAPTER   V. 

THE   EARTH. 

"  The  earth, 

Though  in  comparison  of  heaven  so  small, 
Nor  glistering,  may  of  solid  good  contain 
More  plenty  than  the  sun  that  barren  shines, 
Whose  virtue  in  itself  works  no  effect, 
But  in  the  fruitful  earth ;  there  first  received 
His  beams,  inactive  else,  their  vigor  find." 
e   .  MILTON. 

90.  Dimensions  and  Shape.  —  The  earth  is  a  globular  body,  nearly 
8,000  miles  in  diameter.    The  surface  of  the  ocean  is  not  truly  spher- 
ical, but  bulges  at  the  equator.    If  a  soft  rubber  ball  be  spun  rapidly 
on  an  uncarpeted  floor,  it  will  assume  a  form  like  that  of  the  earth  :  its 
shortest  diameter  will  be  that  on  which,  as  an  axis,  it  rotates.     This 
is  due  to  the  tendency  of  every  particle  of  matter  which  is  whirling 
around  a  centre  to  fly  away  from  it.     Mathematicians  call  the  earth 
an  oblate  spheroid.     Such  a  solid  is  formed  by  revolving  an  ellipse 
(§  96)  about  its  shortest  diameter  as  an  axis. 

91.  Direction  of  the  Plumb-line.  —  A  string,  at  the  lower  end  of 
which  a  plumb-bob  hangs,  is  a  plumb-line.     If  the  earth  were  truly 
spherical  and  homogeneous,   and  did  not  rotate,  its  attraction   for 
the  bob  would   cause  the  line  to  point  directly  to   its  centre.     In 
Fig.  56,  PP'  is  the  earth's  axis,  and  EQ  its  equator.     When  the  earth 
whirls,  the  plumb-bob  at  Q  tends  to  fly  directly  away  from  C,  in  the 
direction  of  the   line   CQ   prolonged.     But  gravity  pulls   the   bob 
directly  toward  C,  and  overcomes  its  tendency  to   fly  away.     At  D, 
on  account  of  the  earth's  rotation,  the  bob  tends  to  fly  away  from 
the  axis  PP',  in  the  direction  indicated  by  the  arrow  A.     This  ten- 
dency causes  the  plumb-line,  instead  of  pointing  toward  C,  to  swing 
a  trifle,  taking  the  position  shown.     At  P,  since  the  plumb-line  is 
in  the  prolongation  of  the  earth's  axis,  the  rotation  causes  no  side- 
wise  swing.     Hence,  at  the  poles  and  at  any  point  on  the  equator  the 


THE    EARTH. 


plumb-line  points  towards  the  earth's  centre  (if  the  earth  be  homoge- 
neous); at  other  places  it  does  not  point  quite  towards  the  centre.  The 
plumb-line  is  perpendicular  to  the  surface  of  still  water. 


Fig.  56.  —  DIRECTION  OF  THE  PLUMB-LINE, 

92.  How  the  Earth's  Diameter  is  Found.  —  While  the  details  of  this 
process  are  too  difficult  for  us  to  understand  at  this  stage  of  our 
progress,  we  may  get  a  notion  of  the  principles  involved  by  assum- 
ing that  the  earth  is  a  perfect  sphere,  and  that  a  plumb-line  points 
toward  its  centre.  In  Fig.  57,  C  is  the  earth's  centre,  and  AB  an 
arc  of  a  meridian.  At  A,  AP  is  drawn  toward  the  north  celestial 
pole,  and  AZ  in  the  direction  of  the  plumb-line.  At  B,  BZ'  is 
drawn  "  plumb,"  and  BD  is  made  parallel  to  AP.  BE  is  parallel  to 
AZ.  Now  the  angle  PAZ  —  DBE,  because  their  sides  are  parallel. 
For  the  same  reason,  — 

EBZ'  =  ACB. 
Then  ACB  =  EBZ', 

=  DBZ'  -  DBE, 
=  DBZ'  -  PAZ. 

5 


66 


DESCRIPTIVE    ASTRONOMY. 


An  astronomer  at  B  measures  the  angle  DBZ';  then,  using  his 
utmost  skill  and  refined  apparatus,  he  determines  the  length  of  AB 
in  feet  or  meters.  On  arriving  at  A,  he  measures  the  angle  PAZ. 
Subtracting  one  of  these  angles  from  the  other,  he  gets  the  value  of 
ACB,  as  explained  above.  If  ACB  were  one  degree,  the  circum- 
ference of  the  sphere  would  be  360  times  the  length  of  AB  in  feet 
or  meters.  Geometry  teaches  that  the  circumference  divided  by 
3.1416  gives  the  diameter. 

The  real  spheroidal  shape  and  the  lengths  of  the  polar  and  equa- 
torial diameters  are  deduced  from  measurements  made  in  various 
parts  of  the  world.1 


Fig.  57- 


Fig.  58.  —  LATITUDE  AND  LONGITUDE. 


93.  Latitude  and  Longitude,  if  the  Earth  were  a  Perfect  Sphere.  — 
Assuming  for  a  moment  that  the  earth  is  a  perfect  sphere,  we  get 
the  following  definitions. 

The  terrestrial  meridian  of  any  point  on  the  earth's  surface  is  the 
circle  drawn  through  the  point  and  the  poles.  In  Fig.  58,  NGG'SR 
is  the  terrestrial  meridian  of  Greenwich,  if  G  represent  that  place. 

The  latitude  of  any  point  on  the  earth  is  the  arc  of  its  meridian, 
between  the  equator  and  the  point.  The  arc  is  measured  in  de- 
grees. A  city  in  30°  north  latitude  is  one  third  of  the  way  from 

1  The  lengths  of  the  diameters  are  respectively  :  — 

Polar 7,901.476  miles. 

Equatorial ...     7,926.592  miles. 


THE    EARTH. 


south  latitude  is  half 


the  equator  to  the  north  pole.     A  place  in  45' 
way  between  the  equator  and  the  south  pole. 

The  longitude  (from  Greenwich)  of  any  point  on  the  earth  is 
the  arc  of  the  equator  embraced  between  the  meridian  of  the  point 
and  the  meridian  of  Greenwich.  It  is  reckoned  either  east  or  west 
of  the  Greenwich  meridian.  Sometimes  it  is  expressed  in  degrees, 
but  more  often  in  hours,  by  putting  360°  equal  to  24  h.  Thus 
15°  equals  I  h. ;  a  place  in  longitude  105°  west  of  Greenwich  is  said 
to  be  7  h.  west  of  Greenwich. 

94.    Latitude    and    longitude    accurately  Defined.  —  Astronomers 
are   accustomed  to  regard  the  earth  as  a  perfect  spheroid  (§  90), 
making    allowances,  when    ne- 
cessary,   for    the    irregularities 
caused  by  the  unevenness    of 
its  surface. 

The  astronomical  latitude  of 
any  point  on  the  earth's  surface 
is  the  angle  made  by  the  plumb- 
line  suspended  at  that  point 
with  the  plane  of  the  equator. 
In  Fig.  59,  the  astronomical 
latitude  of  D  is  the  angle  DAE. 

The  plane  of  the  meridian  of 
any  point  on  the  earth's  surface 
is  the  plane  passing  through 
the  point  and  the  poles.  Any 
two  meridian  planes  intersect, 

their  line  of  intersection  being  the  earth's  axis.  In  Fig.  59,  PF  is 
the  line  of  intersection  of  the  planes  PGP'  and  PEF.  The  angle 
between  these  planes  is  (§  19)  equal  to  ECB,  because  EC  and  BC 
are  both  perpendicular  to  PF.  The  angle  ECB  is  measured  by 
the  arc  EB. 

The  longitude  (from  Greenwich)  of  any  point  on  the  earth's  sur- 
face is  the  angle  made  by  its  meridian  with  the  Greenwich  meridian ; 
or  it  is  the  arc  of  the  equator  lying  between  the  two  meridians. 
Thus,  in  Fig.  59,  the  longitude  of  D,  reckoned  from  G,  is  the  angle 
between  the  planes  PGP'  and  PEP',  which,  as  we  have  found,  is 
measured  by  the  arc  EB. 


Fig.  59. — THE  ASTRONOMICAL  LATITUDE. 


68 


DESCRIPTIVE    ASTRONOMY. 


Latitude  is  measured  in  degrees ;  longitude  in  degrees  or  hours, 
as  stated  in  the  last  section. 

95.  Variation  of  Latitude.  —  During  the  past  few  years  it  has 
been  shown  that  the  latitudes  of  various  observatories  are  not  con- 
stant, but  change  by  slight  amounts,  according  to  a  law  which  has 
been  pretty  thoroughly  determined  by  Dr.  S.  C.  Chandler.1  The 
change  is  due  to  a  "wobbling"  of  the  earth  upon  its  axis,  so  that 
the  north  and  south  poles  do  not  remain  at  the  same  points  on  the 
earth's  surface.  This  movement  of  the  poles  amounts  to  a  few 
yards  in  the  course  of  a  year,  and  takes  place  in  a  very  sinuous 
path.  It  is  believed  to  be  due  to  slight  changes  in  the  earth's 
form,  due  to  the  varying  forces  which  act  upon  it.  A  rude  illus- 
tration is  given  by  a  base  ball,  which  is  usually  rotating  about  some 
axis  just  before  it  is  struck  by  a  bat ;  immediately  after  the  stroke, 
it  commonly  rotates  about  an  entirely  different  axis. 


B 


THE    ECLIPTIC    AND    THE    ZODIAC. 

96.  The  Orbit.  —  The  earth  completes  its  circuit  about  the  sun  in 
a  year,  moving  in  an  ellipse.     This  curve  may  be  drawn  as  shown  in 

Fig.  60.  A  string  runs  from  F  around 
the  pencil  at  P  to  F'.  The  pencil,  being 
moved  in  such  a  way  that  the  string  slips 
around  it  and  is  continually  kept  taut, 
describes  the  curve.  F  and  F'  are  the 
foci.  A  A'  is  the  major  axis ;  BB'  is  the 
minor  axis  ;  C  is  the  centre  The  sun  is 
not  at  the  centre  of  the  earth's  orbit,  but 
at  one  of  the  foci.  When  the  earth  is 
nearest  the  sun,  it  is  said  to  be  in  peri- 
helion ;  when  farthest  away,  in  aphelion.  Half  the  major  axis  is 
called  the  mean  distance  (CA) ;  the  mean  distance  of  the  earth  is 
93,000000  miles.  The  earth  travels  most  swiftly  when  in  perihelion, 
and  with  the  least  velocity  at  aphelion. 

97.  The  Ecliptic.  —  If  the  plane  of  the  earth's  orbit  be  extended 
till  it  cuts   the   celestial   sphere,  their   intersection  will   be  a  circle, 
known  as  the  ecliptic.     In   Fig.  61  is  a  pond  of   water  in  which   a 

1  Of  Cambridge,  Mass. 


Fig.  60.  —  AN  ELLIPSE. 


THE    EARTH. 


69 


croquet  ball  is  floating,  half  submerged.  The  surface  of  the  water 
represents  the  plane  of  the  ecliptic,  and  the  croquet  ball  the  earth, 
the  centre  of  which  is  just  in  the  plane.  The  point  S,  near  the 
centre  of  the  pond,  represents  the  sun's  centre. 

If  the  earth's  axis  were  perpendicular  to  the  plane  of  its  orbit, 
the  planes  of  the  equator  and  ecliptic  would  coincide.  But  the 
axis  is  tipped  so  as  to  make  an  angle  of  23°  27'  with  the  perpen- 


- 


Fig.  61.  —  ILLUSTRATION  OF  THE  ECLIPTIC. 

dicular.  (See  Fig.  62.)  The  plane  of  the  equator,  being  per- 
pendicular to  the  axis,  is  also  tipped,  and  makes  the  same  angle 
with  the  plane  of  the  ecliptic.  This  angle  has  been  named  the 
obliquity  of  the  ecliptic. 

98.  The  Equinoxes.  The  Sun's  Yearly  Path.  —  The  planes  of  the 
ecliptic  and  equator,  when  extended,  cut  the  celestial  sphere  in  two 
circles,  which  cross  each  other  at  two  opposite  points  (A  and  B)  in 
Fig.  63.  The  points  are  the  vernal  and  autumnal  equinoxes  respect- 
ively. It  is  evident,  from  Fig.  61,  that  a  straight  line  drawn  from 
the  earth's  centre  through  that  of  the  sun,  and  prolonged  to  meet 
the  celestial  sphere,  will  strike  at  some  point  of  the  ecliptic.  As  the 
earth  moves  in  its  orbit  around  the  sun,  the  other  end  of  this  line 
moves  along  the  ecliptic. 

To  an  observer  on  the  earth,  the  sun  appears  to  be  on  the  sur- 
face of  the  celestial  sphere,  at  the  end  of  the  line  just  mentioned. 
Hence  the  sun  seems  to  us  to  creep  along  the  ecliptic,  taking  a  year 
to  make  one  complete  circuit.  On  or  near  the  2Oth  of  March,  the 


DESCRIPTIVE    ASTRONOMY. 


sun  is  at  A ;  this  point  is  called  an  equinox,  because  at  that  time 
the  days  and  nights  are  equal,  as  will  be  demonstrated  later.  Six 
months  afterwards,  about  September  22d,  the  sun  is  at  B,  the 
autumnal  equinox. 


Fig.  62. —  THE  OBLIQUITY  OF 
THE  ECLIPTIC. 


Fig.  63.  — THE  ECLIPTIC  AND 
THE  EQUATOR. 


99.  The  Solstices :  The  Sun's  Eastward  Motion.  —  The   point  C  in 
Fig.  63,  midway  between  A  and  B,  is  the  summer  solstice.     The  sun 
is  at  this  point  about  June  2Oth.     In   travelling  from  A,  the  vernal 
equinox,  to  C,  the  summer  solstice,  the  sun   keeps  getting  farther 
away  from  the  celestial  equator  every  day,  and  nearer  to  the  north 
pole.     While  travelling  from  C  to  B,  the  sun  continually  lessens  its 
distance    from    the    celestial    equator.     After    passing    through  the 
autumnal  equinox,  it  is   between    the   south    celestial  pole  and  the 
equator,  and  gets   farther   south   of  the   equator  every  day  until   it 
reaches  D,  the  winter  solstice,  about  December  2ist. 

When  going  from  D  to  A,  it  is  approaching  the  equator  every 
day.  It  must  be  diligently  remembered  that  during  the  entire  year, 
the  sun,  whether  north  or  south  of  the  celestial  equator,  is  contin- 
ually moving  eastward  along  the  sphere.  If  one  could  look  right 
past  the  sun  and  see  the  stars  beyond,  the  amount  of  the  sun's 
motion  among  the  stars  in  a  day  would  be  very  plain  to  the  naked 
eye.  Fig.  64  shows  such  a  view  of  the  sun's  daily  motion. 

100.  The  Zodiac. — The  zodiac  is  a  belt  16°  broad,  encircling  the 
sky,  like  the  colored  band  on  a  croquet  ball.     The  ecliptic   is  its 
central  line.     Twelve  constellations  lie  in  this  belt  or  zone.     They 
are :    Aries,  Taurus,  Gemini ;   Cancer,  Leo,  Virgo ;   Libra,  Scorpio, 
Sagittarius ;   and  Capricornus,  Aquarius,  Pisces.    The  moon  and  the 


THE    EARTH.  71 

planets  are  always  to  be  found  in  the  zodiac.  The  signs  of  the 
zodiac  are  twelve  arbitrary  divisions  of  it,  each  of  which  is  30°  long. 
The  first  three  of  them  stretch  from  the  vernal  equinox  to  the 
summer  solstice.  The  signs  have  the  same  names  as  the  constella- 


ECLIPTIC 


Fig.  64.  — THE  SUN'S  DAILY  MOTION  AMONG  THE  STARS. 

tions  just  mentioned,  but  do  not  coincide  with  them.  The  expres- 
sion, "  The  sun  enters  Aries,"  found  in  almanacs,  means  that  the 
sun  passes  the  vernal  equinox. 

101.  Is  the  North  Celestial  Pole  Fixed?  —  When  a  heavy  top  spins 
rapidly  on  a  smooth  surface,  its   axis  keeps  the  same  direction,  no 
matter  how  much  the  top  wanders  around.     The   successive   posi- 
tions taken  by  the  axis  of  the 

top  are  parallel  lines.  As  the 
earth  goes  spinning  around  the 
sun  once  a  year,  all  the  posi- 
tions of  its  axis  from  moment 
to  moment  are  parallel  lines. 

._.     .  .  *•••-.«  Fi§-  65.  — A  SPINNING  TOP. 

We  here  neglect  certain  minute 

tippings  caused  by  the  attraction  of  other  bodies  for  the  earth. 
These  parallel  lines  when  extended  appear  to  strike  the  celestial 
sphere  at  the  same  point,  because  of  the  infinite  distance  of  the 
sphere  from  us.  This  explains  why  the  north  celestial  pole  can  be 
found  at  the  same  place  among  the  stars  (§  18)  during  the  entire 
year.  It  does  not  move  enough  during  his  lifetime  to  attract  the 
attention  of  one  observing  with  the  naked  eye. 

102.  Are  the  Celestial  Equator  and  Ecliptic  Fixed  ?  —  Since  the  plane 
of  the  earth's  equator  is  perpendicular  to  its  axis,  all  the  positions 


DESCRIPTIVE    ASTRONOMY. 


which  it  takes  during  a  year  will  constitute  a  series  of  parallel  planes. 

These  planes,  being  extended  to  the  celestial  sphere,  cut  it  in  great 

circles,  which  appear  to  us  to 
coincide  on  account  of  their 
infinite  distance  from  us.  Since 
the  earth's  axis  is  tipped  a  little 
this  way  and  that,  as  mentioned 
in  the  last  article,  the  equator 
suffers  slight  shiftings.  But  for 
purposes  of  naked  eye  obser- 
vation, we  regard  the  celestial 
Its  place  among  them  is  shown 


Fig.  66.  —  SUCCESSIVE  POSITIONS  OF  THE 
EARTH'S  EQUATOH. 


equator  as  fixed  among  the  stars, 
on  the  maps. 

Similarly  the  ecliptic  is  to  be  regarded  as  a  circle  practically 
fixed  among  the  stars. 

DAY  AND   NIGHT:  THE  SEASONS. 

103.  The  Length  of  The  Day.  —  Everybody  knows  that  in  summer 
the  days  are  long  and  the  nights  short,  and  that  the  case  is  reversed 
in  winter.  We  refer  now  to  middle  latitudes,  leaving  the  polar 
regions  and  the  equator  out  of  consideration  for  the  moment. 

One  may  get  a  clear  grasp  of  this  matter  by  a  simple  experi- 
ment. Take  an  orange,  with  a  knitting  needle  thrust  through  it  to 
serve  as  an  axis  of  rotation,  and  mark  upon  it  with  a  penknife  the 
equator,  together  with  three  or  four  circles  on  each  side  of  the 
equator  and  parallel  to  it.  Submerge  half  the  orange  in  a  basin  of 
water  in  the  position  shown  in 
Fig.  67.  The  surface  of  the 
water  represents  our  horizon, 
and  the  upper  part  of  the  or- 
ange the  half  of  the  celestial 
sphere  visible  to  us  at  any  mo- 
ment, PP'  being  its  axis  and  EQ 
its  equator.  It  is  plain  that  just 


Fig.  67.  —  AN  ORANGE  HALF  SUBMERGED. 


one  half  of  the  equator  is  below 
the  horizon.     Hence,  if  any  ce- 
lestial object  is  on  the  equator,  as  the  sphere  turns,  the  object  will 
be  above  the  horizon  for  twelve  hours  and  below  for  the  same  length 


THE    EARTH.  73 

of  time.  More  than  half  of  every  circle  between  P  and  EQ  is  above 
the  horizon.  Therefore,  if  any  celestial  object  be  north  of  the 
equator,  it  will  be  above  the  horizon  more  than  twelve  hours  out 
of  the  twenty-four.  If  it  be  near  the  north  celestial  pole,  it  will 
be  above  the  horizon  all  the  time.  Less  than  half  of  every  circle 
between  P'  and  EQ  is  above  the  horizon.  Consequently  any  celestial 
object  south  of  the  equator  is  above  the  horizon  less  than  twelve 
hours  out  of  the  twenty-four.  If  it  be  near  the  south  celestial  pole, 
it  will  never  rise  above  the  horizon. 

On  March  2Oth  the  sun  is  near  the  vernal  equinox  all  day ;  and 
since  this  point  is  on  the  equator,  it  rises  nearly  at  the  east  point  of 
the  horizon  and  sets  nearly  at  the  west  point,  so  that  both  day  and 
night  are  almost  exactly  twelve  hours  long.  After  March  2Oth  the 
sun,  creeping  along  the  ecliptic,  gets  farther  and  farther  north  every 
day  until  June  2Oth,  so  that  the  days  continue  to  grow  longer  and  the 
nights  shorter  until  that  date,  which  is  the  longest  day  of  the  year. 

After  June  2Oth  the  sun  approaches  the  equator  and  the  days 
shorten  and  the  nights  lengthen  until  September  22d,  when  the  sun 
reaches  the  autumnal  equinox,  and  the  days  and  nights  are  again 
equal.  The  sun  then  passes  south  of  the  equator,  and  the  days 
grow  shorter  till  December  2ist,  when  the  sun  is  farthest  south. 
The  sun  approaches  the  equator  during  the  next  three  months,  so 
that  the  days  continually  lengthen  until  the  days  and  nights  again 
become  equal. 

104.  Day  and   Night  at  the  Equator.  —  In  Chapter  II.  we  learned 
that,  if  a  man  lived  at  the  equator,  the  celestial  poles  would  be  on 
his  horizon  at  the  north  and  south  points.     The  orange  used  in  the 
last  section  is  now  to  be  placed  in  the  water,  with  the  knitting  needle 
lying  horizontal  in  the  liquid  surface.      Then  every  circle  marked 
on  the  orange,  as  previously  described,  will  be  half  submerged,  and 
the  sun,  wherever  it  may  be  on  the  sphere  on  any  given  day,  will  be 
above  the  horizon  twelve  hours,  and  below  for  the  same  number  of 
hours. 

105.  Day  and  Night  at  the  Poles.  —  The  horizon  of  a  man  at  the 
north  pole  would  be  parallel  to  the  terrestrial  equator.     Both  planes, 
when  extended  to  the  celestial  sphere,  would  seem  to  cut  it  in  the 
same  circle,  the  celestial  equator.     The  north  celestial  pole  would 
be  in  the  zenith.     Therefore,  if  the  sun  were   north  of  the  equator 


74 


DESCRIPTIVE    ASTRONOMY. 


it  would  be  above  the  horizon,  and  if  south  of  the  equator  it  would 
be  below  the  horizon.  For  the  possible  polar  bears  or  seals  in 
that  locality  there  is  continuous  day  from  March  to  September,  and 
night  for  the  remaining  months  of  the  year.  During  much  of  the 
night  there  is  a  strong  twilight,  because  the  sun  is  not  far  below 
the  horizon. 

106.  The  Midnight  Sun.  —  In  Fig.  68,  PP'  is  the  earth's  axis,  and 
EQ  is  the  equator.  O  is  the  situation  of  an  observer  who  is  in  a 
high  northern  latitude.  OA,  drawn  from  the  observer's  position  at 
O,  parallel  to  the  earth's  axis,  is  directed  toward  the  north  celestial 
pole.  OZ  points  toward  the  zenith,  and  NS  represents  the  horizon. 
Suppose  that  the  observer  is  in  70°  north  latitude,  then  the  angle 
OCQ  will  be  70°,  and  OCP  will  be  20°.  But  OCP=  AOZ.  Since 
OA  points  to  the  pole  and  OZ  to  the  zenith,  and  the  angle  AOZ  is 
small,  the  pole  is  near  the  observer's  zenith. 


N 

MIDNIGHT 


E  .< 

•->  .   \^^ 

Fig.  69.  —  THE  MIDNIGHT  SUN. 

The  appearance  of  the  celestial  sphere,  as  seen  by  an  observer  at 
O  is  shown  in  Fig.  69.  NESW  is  the  horizon ;  EQW  is  half  of  the 
celestial  equator ;  P  is  the  north  pole,  and  OP  is  the  apparent  rota- 
tion axis  of  the  sphere.  In  June  the  sun  is  so  far  north  of  the 
equator  that  its  daily  path  through  the  sky,  ABCD,  lies  entirely 
above  the  horizon,  so  that  it  is  visible  even  at  midnight ;  this  may 
be  made  very  plain  by  using  the  orange  of  §  103. 

107.  The  Seasons  in  Middle  Latitudes.  —  We  consider  first  a  place 
at  a  north  latitude  of  about  45°.  The  change  of  seasons  is  due 
principally  to  two  causes,  i.  The  sun  is  above  the  horizon  during 


THE  EARTH. 


75 


more  hours  on  a  day  in  summer  than  on  a  day  in  winter.     2.    The 

sun  heats  the  earth's  surface  at  any  place  the  more  powerfully  the 

higher  up   it  is  in  the 

sky.     This  is  shown  in 

Fig.  70,  where  a  bundle 

of   rays   from   the   sun, 

coming  nearly  vertically 

downward,  heats  up    a 

square    foot,    while    an 

equal    bundle,    striking 

obliquely,    spreads    its 

heat  out  over  a  greater 

surface ;     consequently 

it  heats  each  square  inch         Fis-  ?°-  ~  EFFECT  OF  THE  SLANT 

-    ,  r  OF  THE  SUN'S  RAYS. 

of  the  surface  less.   The 

oblique  rays  also  traverse  a  longer  path  in  the  atmosphere,  and  are 

more  absorbed  by  it,  before  reaching  the  ground. 

108.  The  Seasons  at  the  Equator. — The  change  of  seasons  is  much 
less  marked  than  in  middle  latitudes :  for  the  sun  is  above  the 
horizon  the  same  time  (twelve  hours)  every  day  of  the  year,  and  its 
rays  come  down  nearly  vertically  at  noon,  throughout  the  year. 


THE  PRECESSION  OF  THE  EQUINOXES,  AND  THE  CALENDAR, 

109.   Attraction  of  the  Earth's  Equatorial  Ring. —  Since  the  earth 
bulges  at  the  equator,  we  may  consider  it  as  a  true  sphere,  around 

the  equatorial  regions  of  which  an 
extra  ring  of  matter  has  been  placed. 
This  conception  is  rudely  represented 
in  Fig.  71.  The  attraction  of  the 
earth  and  moon  upon  this  ring-like 
protuberance  cause  the  precession, 
which  we  proceed  to  explain.  Sup- 
pose the  moon  to  lie  in  the  direction 
of. the  arrows,  and  by  its  attractive 

force  to  be  tugging  aw*y  at  the  ring. 
Its  pull  on  the  half  of  the  ring  at  the 
left  (in  the  figure)  of  C  tends  to  tip  the  earth,  so  that  P  will  move 


76 


DESCRIPTIVE    ASTRONOMY. 


to  the  left.  Its  pull  on  the  half  of  the  ring  at  the  right  of  C  tends 
to  tip  the  earth  in  the  other  direction,  so  that  P  will  move  to  the 
right.  But  since  the  left  half  of  the  ring  is  nearer  the  moon  than 
the  right  half,  it  is  pulled  the  more  strongly,  and  therefore  P  will 
move  slowly  toward  the  left.  In  consequence  of  this  attraction, 
the  earth  will  tend  to  turn  slowly  about  an  axis  going  through  C, 
but  perpendicular  to  the  plane  of  the  page  on  which  Figure  71  is 
drawn.  But  meanwhile  the  earth  is  spinning  with  prodigious 
energy  about  the  axis  P  P',  and  the  moon's  attraction  on  the  ring 
produces  only  a  slight  disturbance  of  this  energetic  rotation. 

110.  Illustration  with  a  Top. —  If  a  top,  which  is  not  spinning,  be 
placed  in  a  slanting  position  on  a  floor,  the  force  of  gravity  pulling 
in  the  direction  of  the  arrow  in  Fig.  72,  will  cause  it  to  fall  over;  in 
falling  it  will  move  just  as  if  it  were  turning  about  a  line  AB,  drawn 


Fig.  72. — A  LEANING  TOP. 


Fig.  73.  —  POSITIONS  OF  THE  Axis  OF  THE  TOP. 


on  the  floor,  as  an  axis.  When  the  top  is  spinning  swiftly  in  an 
inclined  position,  it  rotates  about  the  axis  PS,  and  gravity  is  at  the 
same  time  trying  to  make  it  rotate  about  AB.  Any  one  who  has 
watched  a  top  knows  that,  under  these  circumstances,  the  axis  PS 
moves  slowly  around,  taking  the  successive  positions  PS,  PS',  PS'', 
etc.,  Fig.  73.  PZ  is  perpendicular  to  the  floor.  The  more  swiftly 
the  top  spins,  the  slower  is  this  motion  of  PS  around  PZ. 

111.  The  Earth  compared  with  the  Top.  —  Both  spin  rapidly.  The 
force  of  gravity  attempts  to  tip  the  axis  PS  of  the  top.  The  pulls 
of  the  sun  and  moon  on  the  equatorial  protuberant  ring  of  the  earth 
tend  to  tip  its  axis,  PP'.  The  axis  of  £he  top  swings  around  PZ,  a 


THE    EARTH. 


77 


perpendicular  to  the  plane  of  the  floor.  The  earth's  axis  swings 
around  a  perpendicular  to  the  plane  of  the  ecliptic,  as  shown  in 
Fig.  74.  CA  is  perpendicular  to  the  ecliptic ;  PC,  the  earth's  axis, 
takes  successively  the  positions  PC,  P'C,  P"C,  etc.,  25,800  years 
being  required  for  a  complete  journey  around  the  circle  PP'P"P'". 


Fig.  74.  —  THE  PRECESSIONAL  MOTION  OF  THE  EARTH'S  EQUATOR. 

The  point  where  the  equator  cuts  the  plane  of  the  ecliptic  likewise 
moves  around,  taking  the  positions  V  and  V"  successively.  On 
extending  the  planes  of  the  equator  and  the  ecliptic  to  the  sky,  the 
point  V,  where  the  celestial  equator  and  ecliptic  intersect,  becomes 
the  vernal  equinox. 


DESCRIPTIVE    ASTRONOMY. 


DENEB 


POLARIS 


Hence,  the  vernal  equinox  moves  slowly  along  the  ecliptic,  west- 
ward, taking  25,800  years  for  a  complete  revolution.  The  autumnal 
equinox  does  likewise.  This  is  the  precession  of  the  equinoxes. 

This  motion  of  the  equinoxes  may  be  made  plain  by  using  the 
orange  described  in  §  103,  and  representing  the  ecliptic  by  a  sheet 
of  paper  in  which  a  circle  a  little  larger  than  the  orange  has  been 
cut.  Then,  by  moving  the  orange  in  a  way  suggested  by  Fig.  74, 
the  motion  of  the  equinoxes  becomes  easily  visible. 

112.  Some  Effects  of  Precession.  —  On  account  of  precession  the 
north  celestial  pole,  moving  as  described  in  the  preceding  section, 
comes  near  different  stars  as  the  centuries  pass  away.  About 

twelve  thousand  years 
hence,  the  north  pole 
will  be  so  near  the 
brilliant  star  Vega, 
in  the  constellation 
of  the  Lyre,  that  it 
will  be  called  the  pole 
star. 

The  path  of  the 
pole  among  the  stars 
is  shown  in  Fig.  75. 
Polaris  will  be  only 
half  a  degree  from 
the  pole  in  the  year 

2000. 

The  non-coinci- 
dence of  the  sign  Aries  with  the  constellation  Aries  is  due  to 
precession.  For  the  sign  Aries  begins  at  the  vernal  equinox, 
which,  as  we  have  seen,  shifts  slowly  westward  among  the  stars. 
It  is  now  in  the  constellation  Pisces.  If  Greenwich  and  its  merid- 
ian moved  perceptibly  on  the  earth's  surface,  the  longitudes  of  all 
other  cities  would  be  chajiged.  If  the  earth's  equator  kept  shift- 
ing its  position  on  the  surface,  the  latitudes  of  cities  would  keep 
changing.  In  the  same  way  the  shifting  of  the  celestial  equator 
and  vernal  equinox,  by  precession,  causes  changes  in  the  right 
ascensions  and  declinations  (which  correspond  to  longitudes  and 
latitudes  of  cities)  of  the  stars. 


Fig.  75.  —  THE  PATH  OF  THE  NORTH  CELESTIAL 

POLE   AMONG   THE    STARS. 


THE    EARTH. 


79 


Since  the  vernal  equinox  moves  a  little  westward  every  year, 
the  sun  in  his  apparent  annual  march  eastward  reaches  the  vernal 
equinox  sooner  than  if  it  were  fixed.  This  makes  the  year  twenty 
minutes  shorter  than  it  would  be  otherwise. 

113.  Different  Kinds  of  Years.  —  The  sidereal  year  is  the  time 
required  for  the  earth  to  make  a  complete  revolution  about  the  sun. 
If  the  sun,  as  seen  by  us,  is  now  in  line  with  some  fixed  star,  a 
sidereal  year  must  elapse  before  it  will  get  into  lin£  with  the  same 
star  again.  The  length  of  the  year  is  365  d.  6  h.  9  m.  9  sec. 


STAR 


Fig.  76.  —  DIFFERENT  KINDS  OF  YEARS. 

The  tropical  year  is  the  time  which  elapses  between  two  suc- 
cessive passages  of  the  sun  through  the  vernal  equinox.  Suppose 
that  the  vernal  equinox,  when  the  sun  appears  to  be  in  it,  in  March, 
1900,  is  at  V  in  Fig.  76.  As  the  earth  moves  from  E'  to  E",  E'", 
etc.,  the  sun  appears  to  move  from  V  through  A  to  B,  and  so  on. 
But  meanwhile  the  vernal  equinox,  because  of  precession,  has  been 
moving  westward  a  slight  distance,  so  that  the  sun  will  meet  it  in 
March,  1901,  at  V. 


8O  DESCRIPTIVE    ASTRONOMY. 

A  sidereal  year  will  be  completed  when  the  sun  reaches  V  again. 
A  tropical  year  is  therefore  shorter  than  a  sidereal  year.  Its  length 
is  365  d.  5  h.  48  m.  46  sec. 

114.  The  Julian  Calendar.  —  Julius  Caesar  found  the  Roman  calen- 
dar in  great  confusion.     It  was  decidedly  complex,  and  the  priests, 
whose  duty  it  was  to  regulate  the  religious  festivals  in   accordance 
with   it,  sometimes  introduced   alterations  capriciously,  to   subserve 
their  own  interests.     Matters  had  come  to  a  pretty  pass,  in   Roman 
eyes,  when   a   festival  of  Bacchus  must  be  celebrated  while  grapes 
were  green.     Acting  upon  the  advice  of  Sosigenes,  a  noted  Alexan- 
drian astronomer,  Csesar  ordained  that  three  years  out  of  every  four 
consist  of  365  days,  the  fourth  being  366  days  long.     He  did  this 
because  the  year  was  known  to  be  about  365^  days  in  length.    If  the 
number  of  a  year  be  divisible  by  4  it  is  a  leap  year,  and  contains  366 
days  according  to  the  Julian  calendar.     He   also   directed   that  the 
year  begin  on  Jan.  I,  instead  of  in  March,  as  before. 

This  Julian  calendar  went  into  effect  in  45  B.  c.,  and  is  still 
employed  in  Russia.  Dates  reckoned  according  to  it  are  now 
called  "  Old  Style." 

115.  The  Gregorian  Calendar  — The  true  tropical  year  being  365  d. 
5  h.  48  m.  46  sec.  in  length,  the  Julian  year  of  365  d.  6  h.  is  too  long 
by  ii  m.  14  sec.     This  discrepancy  amounts  to  over  3  days  in  400 
years.       In   1582   Pope  Gregory  XIII.  introduced   a   reform  which 
dropped  3  days  in  every  four  centuries.     This  was  accomplished  by 
ordering  that  the  year  which  rounds  out  each  century  ( 1 300,  1 800,  etc.) 
should  be  a  leap  year  only  when  its  date  number  is  divisible  by  400. 
Thus  1600  was  a  leap  year  according  to  this  calendar,  while  1,700 
was  not.     Accordingly  three  out  of  every  four  century  years   are 
not  reckoned  as  leap  years,  and  3  days  are  thus  dropped  from  the 
Gregorian  calendar  which  are  retained  in  the  Julian.     At  the  time  of 
the  famous   Council  of  Nice   (A.  D.  325)  the  sun  was  in  the  vernal 
equinox  on  March  2ist;  but  in   1582  the  same  event  happened  on 
March   nth,  because   of   the   imperfection  of   the  Julian  calendar. 
The  Pope  therefore  ordered  that  the  10  days  lost  should  be  made 
up  by  calling  the  day  following  October  4th  October  I5th. 

The  new  calendar  was  at  once  adopted  by  all  nations  which 
recognized  the  Pope's  authority.  In  England,  the  Old  Style  was 
used  until  1752,  when  by  act  of  Parliament  the  New  Style  was  intro- 


THE    EARTH. 


8l 


duced,  in  the  face  of  opposition  and  noting  on  the  part  of  some 
of  the  people,  who  acted  as  though  they  believed  that  by  the  change 
of  date  from  September  3d  to  September  I4th,  eleven  days  were  to 
be  stolen  from  them.  It  is  now  quite  common  in  Russia  to  write  a 
date  according  to  both  styles :  thus,  Mar.  3/i5. 

ABERRATION    AND    REFRACTION. 

116.  Aberration  of  Light.  —  One   who  walks    briskly  along   the 
street,   when  the   rain   is   descending  vertically,  does   not  hold   his 
umbrella   straight  up,   but  slants   it  forward.      Were    he  to    stand, 
holding  a  tube   vertical,  raindrops  would   pass   through   it  without 
touching  its  sides.     But  if  he  walked  briskly,  still  holding  the  tube 
upright,  a  drop  of  rain  entering  at  the  top  would  strike   against  the 
side.     However,  if  he  slanted  the  tube  forward  at  the  proper  angle, 
drops   would    go    through    freely. 

While  the  drop  was  failing  the  dis- 
tance AB  in  Fig.  77,  the  man  would 
walk  a  distance  equal  to  CB. 

In  the  same  way,  if  the  earth 
were  still,  and  a  man  pointed  a 
telescope  directly  at  a  fixed  star, 
the  rays  from  the  star  would  come 
down  through  the  telescope  tube 
and  emerge  at  the  eyepiece.  But 
since  the  earth  moves,  it  is  neces- 
sary to  slant  the  telescope  by  a 
minute  angle,  so  that  rays  from 
a  star,  after  passing  through  the 
object-glass,  may  come  out  at  the 
centre  of  the  eyepiece.  Now  the  star  seems  to  us  to  lie  in  the 
direction  in  which  the  telescope  points,  and  a  ray  which  has  the 
direction  AB  appears  to  have  the  direction  AC.  This  apparent 
change  in  the  direction  of  the  ray  is  called  its  aberration.  It  is  a 
quantity  altogether  too  small  to  be  detected  with  the  naked  eye. 

117.  Astronomical  Refraction. — :  In    §  28    we    learned  that  a  ray 
of  light  passing  from  one  medium  into  another   of  different  density 
is  bent  out  of  its  course  unless  it  strikes  the  surface   of  the   new 
medium  perpendicularly. 


77- —  ILLUSTRATION  OF 
ABERRATION. 


82 


DESCRIPTIVE    ASTRONOMY. 


Fig.  78.  —  REFRACTION. 


In  Fig.  78  a  ray  coming  from  S  through  the  earth's  atmosphere  is 
bent  from  its  course.  The  air  may  be  considered  as  made  up  of  a 
large  number  of  strata  of  different  densities,  the  stratum  nearest  the 
surface  of  the  earth  having  the  greatest  density,  because  it  is  com- 
pressed by  the  weight  of  all  the  air  above  it. 
Therefore  the  ray  is  being  continually  bent, 
coming  through  denser  and  denser  strata,  till, 
when  it  reaches  the  eye  at  E,  it  is  coming  in 
the  direction  S'E.  The  star  appears  to  lie 
at  S'  instead  of  S.  Since  S'  lies  nearer  the 
zenith  than  S,  the  effect  of  refraction  is  to 
make  celestial  objects  appear  nearer  the 
zenith,  or  farther  above  the  horizon,  than 
they  really  are.  When  a  star  is  in  the  zenith, 
its  rays  strike  perpendicularly  on  the  atmosphere  and  suffer  no 
deviation.  The  nearer  the  horizon  a  star  is,  the  more  its  rays  are 
refracted.  The  sun  and  the  moon,  when  rising  or  setting,  are  ele- 
vated by  refraction  a  little  over  half  a  degree,  so  that  we  see  them 
when  they  are  really  just  below  the  horizon. 

118.  Twilight.  —  In  the  air  there  are  not  only  clouds,  but  also 
minute  particles  of  dust  and  globules  of  water,  which  reflect  the 
sunlight.  For  some  time  after 
the  sun  has  set,  he  still  shines 
on  the  clouds  over  our  heads, 
and  on  the  particles  in  the 
upper  strata  of  the  air.  The 
light  is  reflected  from  these 
down  to  the  ground,  and  thus 
produces  twilight.  When  the 
sun  gets  about  18°  below  the 
horizon,  his  rays  are  shot  above 
the  clouds  and  other  reflecting 
objects,  so  that  they  are  no 
longer  reflected  to  us,  and  twi- 
light ceases.  In  England  and 
other  countries  of  like  northern 


Fig.  79.  —  (See  Exercise  i.) 


latitude,  twilight  lasts  all  night  in  midsummer,  because  the  sun  is 
less  than   18°  below  the  horizon,  even  at  midnight. 


THE    EARTH.  83 


EXERCISES. 

119,  i.  Suppose  the  earth  to  be  a  perfect  and  smooth  sphere 
and  a  body  to  be  placed  on  it,  as  shown  in  Fig.  79 ;  gravity  would 
pull  it  toward  the  centre,  while  it  would  tend  to  fly  off  in  the  direc- 
tion of  the  arrow  F,  because  of  the  earth's  rotation. 

(a)  As  a  result  of  this  state  of  affairs,  would  the  body  slide 
toward  the  equator? 

(d)  If  the  earth  were  a  perfect  sphere,  would  its  rotation  cause 
the  water  of  the  ocean  to  move  toward  the  equator? 

(c)  Because  of  the  bulge  of  the  earth,  is  the  surface  of  the  water 
at  the  mouth  of  the  Mississippi  River  nearer  the  earth's  centre  than 
at  St.  Paul,  or  farther  from  it? 

(y)  If  the  earth  did  not  rotate,  which  way  would  gravity  make 
the  river  flow? 

(e)  Why  does  the  river  run  south? 

(y)  Why  does  not  the  water  of  the  ocean  leave  the  poles  and 
rush  to  the  equator? 

(g)  If  the  earth  were  composed  entirely  of  water,  and  did  not 
rotate,  what  would  be  its  form? 

2.  If  the  earth  were  a  sphere  the  diameter  of  which  was  7,920 
miles,  wrhat  would  its  circumference  be  according  to  §  92  ?     Ans. 
24,881.472  miles. 

3.  In  Fig.  60,  if  the  sun  be  at  F,  what  points  are  perihelion  and 
aphelion? 

4.  In  Fig.  60,  the  sun  being  at  F,  prove  that  the  semi-major  axis 
(AC)  is  half  the  sum  of  the  greatest  and  least  distances  (A'F  and 
AF)  of  the  earth  from  the  sun. 

5.  Why  does  the  earth  move  most  rapidly  in  its  orbit  when  at 
perihelion? 

6.  Cut  two  equal  circles  out  of  stiff  paper ;   in  each  cut  a  narrow 
slit  from  a  point  on  the  circumference  to  the  centre.     Then  fit  the 
circles  together  in  such  a  way  that  they  will  represent  the  relative 
positions  of  the  planes  of  the  equator  and  ecliptic. 

7.  Let  the  surface  of  a  small  round  body,  like  an  apple  or  orange, 
represent  the  celestial  sphere.    On  it  mark  in  their  proper  positions 
the  north  and  south  celestial  poles,  the  equator,  the    ecliptic,  the 


84  DESCRIPTIVE    ASTRONOMY. 

equinoxes,  and  the  solstices.  Also  mark  the  circle  in  which  the 
north  celestial  pole  makes  its  precessional  journey  of  25,800  years. 
Does  the  south  celestial  pole  describe  a  similar  circle  on  the  sphere? 

8.  The  sun,  when  at  C  or  D  in  Fig.  63,  ceases  to  recede  from  the 
equator  and  begins  to  approach  it.     Show  that  this  fact  harmonizes 
with  the  derivation  of  the  word  "  solstice." 

9.  Why  is  that  equinox   in  which  the   sun  is    on    March   2Oth 
called  the  vernal  equinox? 

10.  If  the  earth's  axis  lay  in  the  plane  of  the  ecliptic,  and  always 
pointed  directly  at  the  sun,  the  north  pole  being  toward  the  sun,  in 
what  portion  of  the  earth  would  the  sun  never  be  visible? 

11.  If  the  earth's  axis  lay  in  the  plane  of  the  ecliptic,  but  always 
pointed  toward  one  particular  fixed  star  as  the  earth  performed  its 
yearly  journey  around  the  sun,  in  what  part  of  the  earth  would  the 
sun  never  be  seen? 

12.  If  the  earth's  axis  were  perpendicular  to   the   plane   of  the 
ecliptic,  and  the  earth  were  rotating  on  its  axis  and  revolving  about 
the  sun  as   fast  as    at  present,  would  all  of  that  part  of  the  earth 
which  is  in  darkness  at  any  instant  be   in   the  light  twelve  hours 
thereafter  ? 

13.  If  the  earth's  axis  made  an  angle  of  70°  with  the  plane  of 
the  ecliptic,  and  the  earth  rotated  as  at  present,  but  did  not  revolve 
around  the  sun,  is  there  any  part  of  the  earth  in  which  there  would 
be  no  night? 

14.  At  the  moment  when  the  sun  lies  in  the  plane  of  the  equator, 
about  March  2Oth  or  September  22d,  does  its  pull  on  the  equatorial 
protuberance  of  the  earth  tend  to  tip  the  earth's  axis? 

15.  When  a  common  almanac  makes  the  statement,  "  The  sun 
enters  Aries,"  does  it  refer  to  the  sign  or  to  the  constellation? 

1 6.  From  an  almanac  which  gives  the  times  of  rising  and  setting 
of  the  sun,  find  out  the  number  of  hours  of  daylight  on  the  longest 
day  of  the  year,  and  also  on  the  shortest,  at  the  place  for  which  the 
almanac  is  computed. 

17.  When  it  is  summer  in  the  northern  hemisphere,  what  is  the 
season  in  the  southern  hemisphere? 

1 8.  At  the  place  where  you  live  does  the  sun  rise  very  near  the 
east  point  of  the  horizon  in  midsummer  (June  2Oth)  ?     Answer  the 
same  question  for  Christmas  time. 


THE    EARTH.  85 

19.  In  each  of  the  four  figures  here  given,  the  earth  is  repre- 
sented as  illuminated  by  the  sun  at  a  certain  time  of  the  year. 
Determine  the  time  of  year  which  corresponds  to  each  figure. 


Fig.  So. 


Fig.  81. 


Fig.  82. 


Fig.  83. 


20.  If  a  man  were  perched  on  the  top  of  a  balloon,  which  was 
rising  straight  up  through  a  shower  of  rain  falling  vertically,  in  what 
position  would  he  have  to  hold  the  glass  tube  of  §  116  so  that  the 
raindrops  would  go  straight  through  it,  without  touching  the  sides? 
If  the  earth  be  moving  directly  toward  a  certain  star,  would  aberra- 


86  DESCRIPTIVE    ASTRONOMY. 

tion  cause  a  telescope  pointed  at  a  star  to  deviate  a  trifle  from  the 
real  direction  in  which  the  star  lay? 

21.  (a)  Does  refraction  make  sunrise  come  earlier  than  it  other- 
wise would,  or  later? 

{b}    What  is  its  effect  on  the  time  of  sunset? 

(c)  Does  it  lengthen  the  number  of  hours  of  daylight,  or  shorten 
them? 

22.  (a)  When  the  moon  is  seen  near  the  horizon,  which  edge 
(or  limb,  as  it  is  called)   is  lifted  more  by  refraction,  the  upper  or 
lower? 

(#)  What  effect  does  this  have  on  the  apparent  shape  of  the 
moon? 

(c)  Does  refraction  cause  the  moon  to  look  larger,  when  near 
the  horizon,  than  when  high  in  the  heavens? 

23.  At  your  home  does  twilight  last  longer  in  midsummer,  or  in 
midwinter? 

24.  Denver  is  in  longitude   105°  W.  of  Greenwich,  and  in  north 
latitude  39°  40'.    Considering  the  earth  as  a  perfect  sphere,  what  are 
the  latitude  and  longitude  of  the  point  on  the  earth's  surface  which 
is  diametrically  opposite  Denver? 

25.  Two  places  in  England  have  the  same  latitude,  and  differ   i° 
in  longitude.     Two  places  in  Ohio  differ  i°  in  latitude,  but  have  the 
same  longitude.     Are  the  places  in  England  as  many  miles  apart  as 
those  in  Ohio? 


CELESTIAL    MEASUREMENTS. 


CHAPTER    VI. 

CELESTIAL   MEASUREMENTS. 

"  Snatch  me  to  heaven ;  thy  rolling  wonders  there, 
World  beyond  world,  in  infinite  extent, 
Profusely  scattered  o'er  the  blue  immense, 
Show  me  :  their  motions,  periods,  and  their  laws, 

Give  me  to  scan." 

THOMSON. 

120.  Position  of  Points  on  a  Sphere.  —  In  describing  the  posi- 
tions of  points  on  the  surface  of  a  sphere,  a  fundamental  circle  is 
first  assumed,  bisecting  the  sphere.  In  Fig.  84,  ABCD  is  the  funda- 
mental circle.  Great  circles  are  those  whose  planes  pass  through 
the  centre  of  the  sphere.  All  others  drawn  on  the  spherical  surface 
are  small  circles,  GPH  is  a 
small  circle.  Secondary  circles 
are  great  circles  which  are 
perpendicular  to  the  funda- 
mental circle.  All  secondaries 
pass  through  two  points  called 
the  poles  of  the  fundamental 
circle.  E  and  F  are  the  poles 
of  ABCD  ;  AECF  and  EBFD 
are  secondaries. 

The  fundamental  point  is 
a  point  of  reference,  which 
lies  on  the  fundamental  circle. 
In  the  figure,  A  is  chosen  as 
the  fundamental  point. 

In  finding  the  position  of 
a  point  on  a  sphere,  we  first  draw  a  secondary  through  the 
point.  If  P  is  the  point,  EPBFD  is  the  secondary  drawn.  We 
then  find  out  two  things:  first,  how  many  degrees  (measured  on 
the  secondary)  the  point  is  above  or  below  the  fundamental  circle, 
that  is,  the  length  of  PB ;  second,  the  number  of  degrees  in  that 


H 


Fig.  84. — CIRCLES  OF  REFERENCE. 


88  DESCRIPTIVE   ASTRONOMY. 

arc  (AB)  of  the  fundamental  circle  which  lies  between  the  funda- 
mental point  (A)  and  the  secondary  (EPBFD).  This  system  has 
already  been  employed  in  defining  the  latitude  and  longitude  of 
a  place  on  the  earth  (§  93). 

121.  The  Horizon  System.  —  In    this    system   the   horizon    is   the 
fundamental  circle.     As   shown  in  §  21,   the  horizon  of  any  place 
is  the  circle  in  which  the  Celestial  sphere  is  cut  by^i  plane  perpen- 
dicular to  a  plumb-line  suspended  at  the  place.     The  secondaries; 
being  perpendicular  to  the  horizon,  are  called  vertical  circles.     All 
vertical  circles  pass  through  the  zenith  and  nadir  (§§  22,  23). 

The  south  point  of  the  horizon  is  usually  taken  as  the  fundamental 
point. 

The  altitude  of  a  celestial  object  is  the  portion  of  its  vertical 
circle  lying  between  it  and  the  horizon. 

Its  azimuth  is  the  arc  of  the  horizon,  measured  from  the  south 
point  around  towards  the  west,  to  its  vertical  circle. 

The  zenith  distance  of  a  celestial  object  is  the  portion  of  its  vertical 
circle  lying  between  it  and  the  zenith.  In  Fig.  84  the  altitude  of  P 
is  PB,  its  zenith  distance  is  EP,  and  its  azimuth  (if  A  be. the  south 
point  of  the  horizon)  is  ADCB. 

The  celestial  meridian  of  any  place  is  that  vertical  circle  which 
passes  through  the  north  and  south  points  of  the  horizon.  It  is 
divided  by  the  poles  into  two  equal  branches ;  the  upper  branch  is 
that  in  which  the  zenith  lies. 

The  prime  vertical  is  that  vertical  circle  which  passles  through 
the  east  and  west  points  of  the  horizon. 

122.  The  Equator  System.  —  Here    the    celestial    equator   is    the 
/  fundamental   circle.      The   secondaries   are  called   hour  circles  ;   all 

hqur  circles  pass  through  the  celestial  poles. 

The  fundamental  point  is  the  vernal  equinox. 

The  declination  of  a  celestial  object  is  the  portion  of  its  hour 
circle  between  it  and  the  celestial  equator.  When  a  star  is  north  of 
the  equator,  it  is  in  north  declination ;  when  south,  it  is  in  south 
declination.  North  declinations  are  accounted  positive ;  south, 
negative. 

The  right  ascension  of  a  celestial  object  is  the  arc  of  the  celestial 
equator,  measured  eastward  from  the  vernal  equinox  to  the  hour 
circle  on  which  the  star  lies. 


CELESTIAL    MEASUREMENTS. 


89 


R 


Right  ascension  may  be  measured  in  degrees,  but  is  usually 
reckoned  in  hours,  like  longitude,  15°  being  equivalent  to  one  hour. 
The  right  ascensions  and  declinations  of  \h.z  fixed  stars  change  very 
little  from  year  to  year. 

The  north  polar  distance  of  a  celestial  object  is  the  portion  of  its 
hour  circle  lying  between  it  and  the  north  celestial  pole. 

Instead  of  the  vernal  equinox,  another  fundamental  point  is  fre- 
quently taken,  viz.  the  point  where  the  celestial  meridian  of  the 
place  of  observation  cuts  the 
celestial  equator.  Then  the 
hour  angle  of  a  celestial  object 
is  the  arc  of^the  equator  em- 
braced between  the  meridian 
and  the  hour  circle  of  the 
pbject.  Hour  angles  are  reck- 
oned either  east  or  west  of 
the  meridian,  east  hour  angles 
being  accounted  negative : 
they  are  usually  reckoned  in 
hours. 

In  Fig.  85,  NESW  is  the 
horizon,  EQWR  the  celestial 
equator,  PABP'  half  of  the 
hour  circle  of  the  star  A,  and 
V  the  vernal  equinox.  AB  is  the  declination  of  A,  VWQB  its  right 
ascension,  and  OB  its  east  hour  angle.  The  hour  angle  of  W,  the 
west  point  of  the  horizon,  is  Q  W,  which  equals  +6  h. :  that  of  the 
east  point  E  is  QE,  which  equals  — 6h. 

123.  Parallax.  —  The  word  parallax  is  a  broad  term.  In  general, 
it  means  the  difference  in  direction  of  an  object  when  viewed  from 
two  different  standpoints :  it  is  the  angle  formed  by  two  lines  drawn 
from  the  object  to  the  two  standpoints  respectively.  Thus,  the 
parallax  of  the  moon,  as  seen  from  Boston  and  the  Cape  of  Good 
Hope,  is  the  angle  between  lines  drawn  from  the  moon's  centre  to 
these  places. 

Usually  the  earth's  centre  is  regarded  as  one  of  the  standpoints : 
then  the  parallax  of  Venus,  for  example,  as  seen  from  Denver  at 
any  instant,  is  the  angle  made  at  Venus  by  two  lines  drawn  from  its 


Fig.  85.  —  THE  EQUATOR,  HORIZON, 
MERIDIAN,  ETC. 


9o 


DESCRIPTIVE    ASTRONOMY. 


centre  to  Denver  and  the  earth's  centre  respectively.  It  is  the 
angle  DVC  in  Fig.  86.  If  the  object  is  in  the  plane  of  the  hori- 
zon of  the  place  of  observation,  its  parallax  is  called  the  horizon- 
tal parallax.  DV'C  is  the  horizontal  parallax  of  an  object  at  V'. 
If  we  consider  the  earth  as  a  perfect  sphere,  the  angle  V'DC  is  a 

right  angle.     We   know 
'V  the    length    of  DC,   the 

earth's  radius.  When 
the  value  of  the  angle  V 
is  known,  it  is  easy  to 
compute  the  distance 
CV  by  elementary  trig- 
onometry. 

If  D  be  a  point  on  the 
earth's  equator,  CV'D 
is  the  equatorial  hori- 
zontal parallax  of  an 
object  situated  at  V. 
The  parallaxes  of  the  sun,  moon,  and  planets  are  large  enough  to 
be  measurable ;  but  the  fixed  stars  are  so  distant  that  the  angle 
CV'D  is  too  small  to  be  measured.  In  getting  the  parallax  of  a 
fixed  star,  the  two  lines  are  drawn  from  the  star  to  the  earth  and 
sun  respectively.  Stellar  parallax  is  discussed  in  §  350. 


Fig.  86. —PARALLAX. 


TIME. 

124.  The  Year.  —  The  two  principal  kinds  of  years  have  been 
explained  already  in  §  113.     The  principles  of  the  Julian  and  Gre- 
gorian calendars  have  been  set  forth  in  §§  114  and  115. 

125.  Solar   Days. — There  are  two  kinds  of  solar  days,  apparent 
solar  and  mean  solar.     An  apparent  solar  day  is  the  interval  of  time 
which  elapses  between  two  successive  passages  of  the  sun  across  the 
upper   branch  of  the  celestial  meridian  of  any  place.     The   earth 
rotates  on  its  axis  at  a  uniform  rate,  so  that,  if  the  sun  were  fixed 
on  the  celestial  sphere,  all  solar  days  would  be  of ' equal   length. 
But  the  sun   appears  to  creep  slowly  eastward  on  the  sphere,  on 
account  of  the  yearly  revolution  of  the  earth,  and  at  an  irregular 
rate,  creeping  more  on  some  days  than  others,  as  will  be  explained 


CELESTIAL    MEASUREMENTS. 


in  the  next  section.  Hence  apparent  solar  days  vary  in  length,  the 
greatest  variation  being  nearly  a  minute. 

The  sun  being  so  irregular  a  timekeeper,  astronomers  have 
devised  a  fictitious  sun,  called  the  mean  sun,  which  moves  in  the 
equator  at  a  uniform  rate^  completing  its  apparent  journey  around 
the  celestial  sphere  in  a  year.  The  mean  sun  crosses  the  meridian 
sometimes  a  few  minutes  in  advance  of  the  true  sun,  at  other  times 
a  few  minutes  behind  it.  The  greatest  difference  is  sixteen  minutes. 
A  mean  solar  day  is  the  interval  between  two  successive  passages  of 
the  mean  sun  over  the  upper  branch  of  the  celestial  meridian  of  any 
place. 

All  ordinary  clocks  and  watches  are  regulated  by  mean  solar 
time. 

126.  Causes  of  the  Unequal  Lengths  of  Apparent  Solar  Days.  ^- 
There  are  two  principal  causes :  — 

I.  The  earth  in  travelling  around  the  sun  does  not  move  at  a 
uniform    speed.     When    at   perihelion    it   moves    most   swiftly;    at 
aphelion,  with  the  least  velocity.     Since  the  apparent  motion  of  the 
sun  eastward  is  due  to  the  earth's  revolution  about  it,  the  distance 
over  which  the  sun  creeps  each 

day  on  the  sphere  varies,  being 
greatest  when  the  earth's  motion 
is  most  rapid. 

II.  But  even  if  the  sun  moved 
at  a  uniform  rate  in  the  ecliptic, 
apparent  solar  days  would  still 
vary    in   length.     Suppose    that 
the  mean  and  true  suns  are  to- 
gether   at   the    vernal    equinox, 
V,  in  Fig.  87.     After  one  fourth 
of  a  year  has  elapsed,  each  sun, 
if  moving  uniformly,  will   have 

described  an  arc  of  90°  ;  the  true  sun  will  then  be  at  S,  the  summer 
solstice,  and  the  mean  sun  at  Q,  both  lying  on  the  same  hour 
circle,  PSQ. 

Let  M  be  the  middle  point  of  VQ,  and  draw  the  hour  circle  PM 
cutting  the  ecliptic  at  R.  One  easily  sees  that  the  arc  VR  is  longer 
than  VM,  so  that,  when  the  mean  sun  arrived  at  M.  the  true  sun  had 


M 


Fig.  87.  —  UNEQUAL  LENGTHS  OF 
APPARENT  SOLAR  DAYS. 


DESCRIPTIVE    ASTRONOMY. 


not  reached  R.  Hence  the  mean  sun  and  the  true  sun,  though 
together  on  the  same  hour  circle  at  the  beginning  and  at  the  end  of 
their  three  months'  race,  did  not  keep  on  the  same  hour  circle  during 
the  race.  Now  any  celestial  objects  which  are  on  the  same  hour 
circle  cross  the  meridian  at  the  same  time.  Hence,  while  the  mean 
sun  by  its  successive  passages  across  any  given  meridian  was  mark- 
ing off  days  of  uniform  length,  those  marked  off  by  the  true  sun 
were  not  of  uniform  length. 

127.  Another  Explanation.  —  On  a  sphere,  (the  larger  the  better,) 
draw  the  ecliptic  and  equator.  Mark  the  summer  solstice,  and  put 
another  mark,  a  quarter  of  an  inch  away,  on  the  ecliptic.  Draw  an 
hour  circle  through  each.  Place  a  mark  on  the  ecliptic  a  quarter 
of  an  inch  from  the  vernal  equinox  and  draw  hour  circles  through 
these  two  points.  One  sees  at  once  that  the  first  pair  of  hour 
circles  make  a  larger  angle  with  each  other  than  the  second  pair. 

The  quarter  of  an  inch  represents 
the  amount  that  the  sun  creeps  on 
the  ecliptic  in  a  day. 

At  the  time  of  the  vernal  equi- 
nox an  apparent  solar  day  would 
be  the  time  of  a  rotation  of  the 
earth  plus  the  time  required  for  it 
to  turn  through  the  angle  between 
the  first  pair  of  hour  circles. 

At  the  time  of  the  summer  sol- 
stice an  apparent  solar  day  would 
be  the  time  of  a  rotation  of  the 


POLE 


POLE 


ECLIPTIC 


EQUATOR 


Fig. 


!.  — VARIABLE  MOTION  IN 
HOUR  ANGLE. 


earth  plus  the  time  required  for  it 
to  turn  through  the  angle  between 
the  second  pair  of  hour  circles.  Since  the  second  angle  is  larger 
than  the  first,  the  two  days  would  be  of  unequal  length.  Fig.  88 
exhibits  the  arcs  drawn  as  directed. 

128.  A  Sidereal  Day.  —  A  sidereal  day  is  the  interval  between 
two  successive  passages  of  the  vernal  equinox  over  the  upper  branch 
of  the  celestial  meridian  of  any  place.  Since  the  apparent  daily 
rotation  of  the  celestial  sphere  is  caused  by  the  real  rotation  of  the 
earth,  sidereal  days  are  of, uniform  length.  The  length  of  this  day 
might  be  changed  by  various  causes.  A  gradual  shrinkage  of  the 


CELESTIAL    MEASUREMENTS.  93 

earth  in  cooling  would  make  it  rotate  faster :  the  friction  caused  by 
the  tidal  movement  of  the  water  of  the  ocean  tends  to  impede  the 
rotation.  However,  no  change  in  the  length  of  a  day  has  ever  been 
detected  by  observations.  It  is  considered  certain  that  it  has  not 
changed  a  hundredth  of  a  second  in  the  past  thirty  centuries. 

129.  Civil  Day  and  Astronomical  Day.  —  Each  of  these  days  con- 
sists of  24  hours  of  mean  solar  time.     The  civil  day,  employed  for 
the  ordinary  purposes  of  life,  begins  at  midnight.     The  astronom- 
ical day  begins  12  hours  later  than  the  civil  day,  at  noon. 

Thus,  Sept.  3,  9  P.  M.  of  civil  time  is  equivalent  to  Sept.  3,  9  h. 
by  astronomical  time;  but  Sept.  3,  9  A.  M.  of  civil  time  is  Sept.  2, 
21  h.  by  astronomical  time.  An  astronomer  when  observing  at 
night  is  saved  the  trouble  of  changing  the  date  in  his  record-book 
when  midnight  comes.  A  movement  is  on  foot  to  discontinue  the 
use  of  the  astronomical  day  at  the  end  of  the  nineteenth  century. 

130.  Mean    Solar    and    Sidereal    Time.  —  An    astronomical    clock 
keeping  mean   solar  time  at  Denver,   for  instance,  reads  oh.  o m. 
o  sec.,  when  the  mean  sun  is  on  the  upper  branch  of  the  meridian  of 
Denver,  at  noon.     Its  face  is  graduated  from  o  h.  to  23  h.,  and  the 
hour  hand  sweeps  around  the  dial  once  in  a  mean  solar  day.     Such 
a  clock  is  said  to  keep  "  local  "  mean  time. 

A  sidereal  clock,  on  the  other  hand,  reads  oh.  om.  osec.,  when 
the  vernal  equinox  is  on  the  upper  branch  of  the  meridian.  When 
it  reads  roh.  30 m.  osec.,  it  shows  that  the  vernal  equinox  crossed 
the  meridian  loj  sidereal  hours  ago.  A  reading  23  h.  om.  osec. 
indicates  that  the  vernal  equinox  crossed  the  upper  branch  of  the 
meridian  23  sidereal  hours  ago,  and  will  cross  again  in  an  hour. 

If  the  mean  sun  and  the  vernal  equinox  were  together  on  the 
celestial  meridian  of  a  given  place,  both  the  sidereal  and  the  mean 
time  clock  at  that  place  should  indicate  o  h.  o  m.  o  sec.  After  a 
lapse  of  24  sidereal  hours,  the  vernal  equinox  would  be  on  the 
meridian  again ;  but  the  mean  sun,  on  account  of  its  eastward 
motion  on  the  sphere,  would  be  east  of  the  vernal  equinox,  and 
would  cross  the  meridian  a  little  while  after  it.  A  mean  solar  day 
is  therefore  longer  than  a  sidereal  day.  As  the  mean  sun  travels 
entirely  around  the  sphere  in  a  year,  in  a  day  it  moves  about  o-J^  of 
the  circumference  of  the  sphere,  making  the  mean  solar  day  ^.^  h., 
or  nearly  four  minutes,  longer  than  the  sidereal  day. 


94  DESCRIPTIVE    ASTRONOMY. 

131.  Right  Ascension  vs.  Sidereal  Time.  —  By    the    definition    of 
right  ascension   (§  122),  we  see  that  a  star  whose  right  ascension 
is  4  h.  will    cross  the    meridian  of  any   place   4  h.  after   the  vernal 
equinox  has  crossed  the   same   meridian.     But   the    sidereal   clock 
will  read  4  h.     We  therefore  have  the  following  principle. 

The  right  ascension  of  any  star  is  equal  to  tJie  sidereal  time  at  any 
place  at  the  instant  when  the  star  is  on  the  meridian  of  that  place. 

Thus  when  a  star  whose  right  ascension  is  exactly  16  h.  is  on 
the  meridian  of  the  U.  S.  Naval  Observatory  at  Washington,  the 
Washington  sidereal  clock  should  read  16  h.  o  m.  o  sec. 

When  the  sidereal  time  is  13  h.,  the  star  mentioned  has  an  east 
hour  angle  of  3  h.,  because  3  h.  must  elapse  before  it  will  reach  the 
meridian.  If  the  sidereal  time  be  18  h.,  the  star  crossed  the  meridian 
two  hours  before,  and  hence  has  a  west  hour  angle  of  2  h. 

132.  Relation  between  Longitude  and  Time.  —  If  the  longitude  of 
a  place  is  one  hour  west  of  Greenwich,  the  mean  sun  arrives  at  its 
meridian   one  hour  after  it    has  crossed    the  Greenwich  meridian ; 
hence  at  noontime  at  the  place  in  question  it  is   I  P.  M.  at  Green- 
wich.    When  a  city  is  two  hours  east  of  Greenwich,  the  sun  crosses 
its  meridian  two  hours  before  it  reaches  the  meridian  of  Greenwich ; 
when  it  is  10  A.  M.  at  Greenwich,  it  is  noon  at  the  city. 

133.  Where    the    Date    Changes.  —  A    place   whose    longitude    is 
1 80°,  or   1 2  h.  west  of  Greenwich,  is  also   I2h.  east  of  Greenwich. 
Reckoning  it  as  12  h.  west  of  Greenwich,  its  time  will  be   12  h.  less 
than  the  Greenwich  time ;    so  that  when  it  is   1 1   A.  M.  on  July  4 
at  Greenwich,  it  will  be   1 1   P.  M.  on  July  3  at  this  place.     But  if 
we   say  that   the    place   is    12  h.   east  of  Greenwich,   its    time   will 
be   I2h.  more  than  the  Greenwich  time  at  the  same  instant,  mak- 
ing  ii  P.  M.  of  July  4.     Thus  there   is  a  discrepancy  of  one  day, 
according  as  we  count   east  or  west    from    Greenwich.     Mariners 
when  crossing  the  iSoth  meridian  change  the  date;   in  going  west, 
an  entry   in   the  log-book  just  before   crossing  the  line  might  be 
dated  Wednesday,  Sept.    12,  3  P.M.     An  entry  made  an  hour  after- 
ward, if  the  ship  had  crossed  the  line  meanwhile,  would   be  dated 
Thursday,  Sept.    13,  4P.M.     Similarly,  in  crossing  from  the  west, 
Tuesday,  October  9,  would  be  changed  to  Monday,  October  8. 

134.  Standard  Time.  —  There    is   now  in    use    throughout   North 
America  a  system  of  standard  time,  which  is  a  great  boon   to  the 


CELESTIAL    MEASUREMENTS. 


95 


business  world.     Five  standard   meridians  have  been  adopted,  west 

of  Greenwich  4,  5,  6,  7,  and  8  hours  respectively.     A  city  generally 

adopts  the  time  of  that  standard  meridian  to  which  it   is  nearest. 

The  standard  times  are  called  respectively  Colonial,  Eastern,  Central, 

Mountain,  and  Pacific.    In  a  few  large  cities  there  is  still  confusion.    At 

Pittsburg  both  Eastern   and   Central  times 

are  in  use.     It  would  be  well  if  all  towns 

in    the    same    State  kept    the    same    time. 

Several  European  countries   have  adopted 

standard  times  based  upon  the  meridian  of 

Greenwich. 

135.  Clocks  and  Watches.  —  Accurate  time 
obtained  from  various  astronomical  obser- 
vatories (chiefly  from  Washington)  is  tele- 
graphed  daily  all  over  the  United  States. 
The  clocks  in  the  observatories  are   regu- 
lated with  the   greatest  care,  so  that  they 
are    rarely    over    a  second    in   error.     The 
pendulum  jar  of  the  clock  shown  in  Fig.  89 
is    nearly    filled   with    mercury.     In    warm 
weather  the   pendulum  rod    lengthens    be- 
cause of  the  heat;   this  lengthening  would 
make  the  clock  go  slow,  but  the  mercury 
expanding  rises  in  the  jar,  and  thus  coun- 
teracts the  elongation  of  the  rod.     In  cold 
weather   there    is   a  similar  compensation : 
such  a  clock  is  said  to  be  compensated  for 
temperature. 

Good  watches  should  be  wound  regularly 
and  kept  in  the  same  position  by  day  and 
by  night.  When  the  second  hand  is  at 
sixty,  the  minute  hand  should  be  over  some 
minute  mark.  A  sudden  change  of  rate  is 
not  a  sure  sign  that  the  regulator  needs  to 
be  moved.  The  best  watches  exhibit  anomalous  variations  at  times, 
and  frequently  right  themselves  without  being  regulated  anew. 

136.  The  Meridian  Circle.  --  The  telescope  of  this  instrument  is 
put  in  the  middle  of,  and  at  right  angles  to,  an  axis  which  turns  in 


—  A  STANDARD 
CLOCK. 


96 


DESCRIPTIVE    ASTRONOMY. 


Fig.  90.  —  A  PORTABLE  MERIDIAN  CIRCLE. 


CELESTIAL    MEASUREMENTS. 


97 


a  bearing  at  each  end  as  shown  in  Fig.  90.  The  axis  is  horizontal, 
and  points  due  east  and  west.  Upon  the  axis  are  mounted  a  couple 
of  graduated  circles,  which  are  used  for  finding  the  altitude  of  stars 
when  they  cross  the  meridian.  At  the  focus  of  the  object-glass  near 
the  eye-end  of  the  telescope  is  the  "  reticle,"  which,  as  seen  through 
the  eyepiece,  presents  the  appearance  shown  in  Fig.  91.  It  is 
usually  made  of  spider-webs,  or  of  fine  lines  ruled  on  a  thin  piece  of 
glass.  The  "  wire  "  AB  is  parallel  to  the  axis  of  the  instrument. 


Fig.  91.  —  A  RETICLE. 

137.  A  Star  passes  the  Me- 
ridian. —  If  the  telescope  be 
pointed  in  the  direction  of  a 
star  just  before  it  passes  the 
meridian,  it  will  be  seen  to 
move  across  the  field  of  view 
in  a  path  parallel  to  AB,  cross- 
ing each  of  the  five  parallel 
wires.  The  reason  for  this  is 
shown  in  Fig.  92.  A  line  drawn 
from  the  star  at  S  through  the 
centre  of  the  object-glass  meets 
the  star's  image  at  I.  As  the 

its 


Fig.  92.  — MOTION  OF  A  STAR'S  IMAGE. 


star    moves    to    S'    and  S" 

image  moves  to  V  and  V .     Therefore,  the  observer  sees  the  star's 

image  crossing  the  wires  of  the  reticle. 

If  ST  is  perpendicular  to  the  rotation  axis,  it  lies  in  the  plane 

7 


98  DESCRIPTIVE    ASTRONOMY. 

of  the  meridian,  for  the  meridian  plane  is  perpendicular  to  a  hori- 
zontal east  and  west  line.  Therefore  both  the  star  S'  and  its  image 
I'  lie  in  the  plane  of  the  meridian.  The  reticle  is  so  placed  that 
the  star's  image  when  at  I'  lies  on  the  middle  wire  in  Fig.  91. 

138.  Determination  of  Clock  Error.  —  In  the  Nautical  Almanac,1 
which  is  issued  yearly,  is  to  be  found  an  extensive  list  of  bright 
stars,  the  right  ascensions  of  which  are  given.  The  right  ascension 
of  any  star,  as  we  have  learned,  equals  the  sidereal  time  when  that 
star  crosses  the  observer's  meridian.  The*  astronomer  who  wishes 
to  find  the  error  of  his  sidereal  clock  selects  from  the  list  a  star, 
Sirius,  for  example,  and  points  the  telescope  of  the  meridian  circle 
in  such  a  direction  that  Sirius  when  it  crosses  the  meridian  will 
pass  through  the  field  of  view.  As  it  crosses  each  wire  of  the 
reticle  he  notes  the  reading  of  the  clock  as  below:  — 


h. 

6 

m. 

39 

sec. 
46.3 

39 

56.2 

40 

II.4 

40 

26.6 

40 

36.6 

6 

40 

11.42 

Average, 

The  average  of  these  readings  is  a  pretty  accurate  value  of  the 
clock  reading  when  the  star  crossed  the  middle  wire,  which  repre- 
sents the  meridian.  Turning  to  the  almanac,  he  finds  that  on  the 
date  of  observation  the  right  ascension  of  Sirius  was  6  h.  40  m. 
26.94  sec.  Hence,  if  the  instrument  was  in  perfect  adjustment,  the 
error  of  the  clock  was  15.52  sec.  Was  the  clock  fast  or  slow  ? 

To  attain  greater  accuracy,  the  astronomer  observes  a  number 
of  stars.  Four  stars,  after  allowance  had  been  made  for  errors  in 
the  position  of  the  meridian  circle,  might  give  him  for  the  clock 
error  respectively  15.52  sec.,  15.46  sec.,  15.53  sec.,  and  15.45  sec.  The 
average  of  these  is  15. 49  sec. 

By  comparing  observations  made  on  two  dates,  the  amount  that 
a  clock  gains  or  loses  in  a  day  can  be  found.  This  amount  is  the 

1  A  copy  of  this  work,  if  not  obtained  through  a  Senator  or  Representative,  may  be 
had  by  sending  one  dollar  to  the  Nautical  Almanac  office,  Washington,  D.  C.  The 
British,  German,  French,  and  some  other  nations  publish  such  almanacs. 


CELESTIAL    MEASUREMENTS. 


99 


daily  rate.     Sidereal  time  is  easily  reduced  to  mean  time  by  tables 
given  in  the  Nautical  Almanac. 

139.  The  Chronograph.  —  A  chronograph  is  employed  to  facilitate 
noting  the  time  when  any  phenomenon  occurs.  Its  most  common 
use  is  to  record  the  reading  of  a  timepiece  at  the  instant  when  an 
astronomer  sees  a  star  cross  a  wire  in  the  reticle  of  his  meridian 
circle. 


- 


Fig.  93.  —  A  CHRONOGRAPH. 

A  sheet  of  paper  is  wrapped  around  the  barrel,  which  is  rotated 
once  a  minute  by  the  mechanism,  driven  by  a  weight  not  shown  in 
the  figure.  If  this  were  the  only  motion,  the  pen  which  rests  on 
the  barrel  would  draw  a  circle  around  the  cylinder  in  a  minute,  and 
repeat  the  same  circle  the  next  minute,  and  so  on,  as  long  as  the 
mechanism  ran.  But  the  carriage  which  holds  the  pen  is  mounted 
on  a  long  screw,  shown  in  front  of  the  barrel :  the  screw  rotates  once 
a  minute,  and  continually  moves  the  pen  carriage,  which  is  set  near 
one  end  of  the  barrel  at  starting,  toward  the  other  end  of  the  barrel. 
Consequently  the  pen,  instead  of  making  a  single  circle  over  and 
over,  makes  a  long  spiral  line,  like  a  screw  thread,  running  from 
one  end  of  the  barrel  to  the  other. 


IOO 


DESCRIPTIVE    ASTRONOMY. 


140.  Records  of  a  Clock  and  Key.  —  By  suitable  electrical  connec- 
tions, which  cannot  well  be  explained  here,  a  clock  causes  the  pen 
to  give  a  quick  vibration  each  second,  so  that  a  series  of  notches 
are  made  in  the  line,  as  shown  in  Fig.  94.  No  notch  is  made  at  the 


/6        /5        /**• 


I/         10         9          8          7         6         S         4*         3         2. 

Fig.  94.  —  A  CHRONOGRAPHIC  RECORD. 


fifty-ninth  second  of  each  minute :  the  first  notch  thereafter  marks 
the  beginning  of  a  new  minute.  The  observer  notes  the  time  which 
the  clock  read  when  some  particular  notch  was  made,  and  marks  that 
notch.  In  Fig.  94  the  notch  for  9  h.  6  m.  o  sec.  was  thus  marked  : 
from  this  record  he  can  tell  the  time  when  any  other  notch  was  made 
by  the  clock. 

The  astronomer,  when  observing,  has  at  his  side  a  telegraph  key, 
connected  with  the  chronograph :   when  he  wishes  to  note  the  time, 

he  presses  the  key  quickly,  and  the 
pen  makes  a  notch  at  that  instant. 
Fig.  94  shows  some  records  made 
between  9  h.  I  m.  and  9  h.  6  m.,  on  a 
small  portion  of  a  sheet.  In  the 
place  where  the  fifty-ninth  second  of 
each  minute  is  omitted,  the  observer 
has  written  on  the  lines  the  numbers 
of  the  successive  minutes.  The 
numbers  of  the  seconds  of  each  min- 
ute are  written  along  one  of  the 
lines.  There  are  three  extra  marks, 
made  when  the  observer  pressed  his 
key.  One  of  the  marks  was  made 
at  9h.  I  m.  4.5  sec.,  another  at  9h. 
3  m.  9.4  sec.,  and  the  third  at  Qh. 
4m.  13. 8  sec. 

141.   Determination  of  Latitude.  —  Let  D  in  Fig.  95  be  a  point  on 
the  earth's  surface,  NS  its  horizon,  P'P"  the  earth's  axis,  and  EQ 


Fig.  95- 


CELESTIAL    MEASUREMENTS. 


101 


the  equator.  ZDO  is  perpendicular  to  NS.  PD  is  parallel  to  FP", 
and  therefore  points  to  the  north  pole  of  the  celestial  sphere.  DT 
is  parallel  to  EO.  PDT  is  a  right  angle.  By  definition  (§  94) 
the  latitude  of  D  is  DOE.  This  equals  ZDT  =  90°  -  ZDP  =  PDN, 
which  by  definition  (§  12 1)  equals  the  altitude  of  the  celestial 
pole.  Hence  the  latitude  of  the  place  of  observation  equals  the  alti- 
tude of  the  pole.  Though  an  astronomer  cannot  see  the  north  celes- 
tial pole,  he  can  find  its  altitude  by  observing  the  Pole  Star  when  it  is 
on  the  meridian.  In  Fig.  96,  D,  is  the  place  of  observation,  NS  the 
horizon,  NZS  the  meridian,  P  trie  north  celestial  pole,  P'  and  P"  the 
two  positions  of  the  Pole  Star  when  it  crosses  the  meridian.  By 


M 


Fig.  96.  —  LATITUDE  FOUND  BY  OBSERVATION  OF  POLARIS. 

means  of  the  divided  circle  on  the  axis  of  the  meridian  circle,  the 
astronomer  measures  the  angle  P'DN,  the  altitude  of  the  Pole  Star 
when  it  is  on  the  meridian  above  the  pole.  Twelve  hours  later  he 
measures  P"DN. 

P'DN  =  PDN  +  PDF 

F'DN  =  PDN  -  PDP" 

FDN  +  P"DN  =  2  PDN  +  PDF  —  PDF'. 
But  PDP'  ==  PDP". 

Therefore  FDN  +  P"DN  =  2  PDN 

PDN  =  $  (FDN  +  P"DN). 
PDN  is  the  latitude  required. 

The  method  by  which  the  angles  P'DN  and  P"DN  are  measured 
will  be  understood  readily  by  examining  an  engineer's  transit,  such 
as  is  shown  in  Fig.  97. 


102 


DESCRIPTIVE    ASTRONOMY, 


.  97.  —  AN  ENGINEER'S  TRANSIT. 


CELESTIAL    MEASUREMENTS. 


103 


142.  Determination  of  Longitude.  —  We  have  seen  (§  132)  that 
the  difference  in  longitude  between  two  places,  such  as  Washington 
and  Chicago,  is  equal  to  the  difference  at  any  instant  between  the 
readings  of  two  clocks,  one  of  which  keeps  correct  Washington  time, 
while  the  other  keeps  Chicago  time.  If  a  chronometer  keeping 
Washington  time  be  carried  to  Chicago  and  compared  with  the 
Chicago  clock,  the  difference  between  their  readings,  if  both  are 


Fig.  98.  —  A  CHRONOMETER. 

correct,  is  the  difference  of  longitude  sought.  In  practice  several 
chronometers  are  used,  the  errors  of  which  —  for  no  chronome- 
ter or  clock  runs  exactly  right  —  are  very  carefully  determined 
by  observations  of  the  stars.  .The  electric  telegraph,  however, 
furnishes  a  much  more  accurate  method.  By  quite  simple  mech- 
anism the  Washington  clock,  as  it  ticks,  makes  a  telegraphic 
sounder  at  Chicago  click,  so  that  an  astronomer  at  Chicago  can 


104 


DESCRIPTIVE    ASTRONOMY. 


compare  the  telegraphic  beats  of  the  Washington  clock  with  his 
own.  The  times  at  which  the  signals  are  to  be  sent  are  agreed 
upon  beforehand. 

143.  The  Position  of  a  Ship.  —  A  mariner  usually  finds  the  lati- 
tude and  longitude  of  his  ship  by  observations  of  the  sun  made 
with  a  sextant,  a  little  instrument  easily  held  in  the  hand.  A 
chronometer  keeping  some  standard  time,  as  that  of  Greenwich, 
is  also  necessary.  In  Fig.  96,  let  M  represent  the  sun  when  on 
the  meridian.  The  mariner  measures  its  altitude,  MS,  with  the 


Fig.  99.  —  A  SEXTANT. 


sextant.  From  the  almanac  he  finds  PM,  the  distance  of  the  sun 
from  the  pole.  The  sum  of  these  gives  PMS,  which,  subtracted 
from  1 80°,  leaves  PM,  the  latitude  desired. 

To  find  the  longitude,  the  altitude  of  the  sun  is  measured  about 
the  middle  of  the  forenoon  or  afternoon,  and  the  reading  of  the 
chronometer,  keeping  Greenwich  time,  is  noted  at  the  same  time. 
Suppose  that  the  measured  altitude  was  62°  15'  20."  By  means  of 
astronomical  tables  the  mariner  computes  the  time  at  which  the 
sun  had  that  altitude;  it  might  have  been  3  h.  i6m.  27  sec.  If 


CELESTIAL    MEASUREMENTS.  1 05 

the  Greenwich  time,  found  from  the  chronometer,1  was  4h.  29m. 
48  sec.,  the  ship  was  evidently  in  longitude  I  h.  1301.  21  sec.  west 
of  Greenwich. 


EXERCISES. 

— 144.    i.  Is  the  arctic  circle  on  the  earth  a  great  circle  or  a  small 
circle? 

2.  Consider  the  earth  as  a  perfect  sphere,  and  the  equator  as  the 
fundamental  circle ;   what  is  the  name  applied  in  geography  to  the 
secondaries? 

3.  Consider  the   earth   as  a   perfect  sphere.     In  estimating  the 
longitudes   of  points  from   Greenwich,  what  point   is  taken   as  the 
fundamental  point? 

-^  4.    (a)  When  the  sun  is  just  rising,  what  is  its  altitude? 
•-(^)    When  a  star  is  in  the  zenith,  what  is  its  altitude? 

5/'(tf)   What  is  the  azimuth  of  a   point  on  the  prime  vertical, 
the  p\>int  being  west  of  the  zenith? 

~(b)    What  is  the  azimuth  of  the  north  celestial  pole? 
-(£•)    What  is  the  azimuth  of  the  east  point  of  the  horizon? 
— 6.    (a)   Does  the  celestial  meridian  of  any  place  pass  through  the 
nadir? 

_(#)    Do  the  celestial  poles  lie  on  this  meridian? 
— (<;)    Is  there  any  point  on  the  earth  where  the  meridian  coincides 
with  the  celestial  equator? 
*-(d)    Does  your  celestial  meridian  cut  the  celestial  equator? 

(V)  If  so,  can  you  point  your  finger  toward  a  point  of  inter- 
section? 

^(/)    Where  does  the  prime  vertical  cut  the  meridian? 
~{g)   What  position  has  the  plane  of  the  prime  vertical  with  refer- 
ence to  that  of  the  meridian  ? 

7.  (a)  The  declination  of  a  star  is  +20°  ;  what  is  its  north  polar 
distance? 

1  The  reading  of  the  chronometer  face  is  not  the  true  Greenwich  time.  Before 
leaving  port  the  error  of  the  chronometer  (usually  a  few  seconds)  and  the  daily  rate 
(§  138)  were  found  by  astronomical  observations.  From  these  the  error  of  the  chro- 
nometer at  any  time  during  the  voyage  can  be  computed,  and  allowance  made  for  it  to 
get  the  true  Greenwich  time.  While  chronometers  do  not  keep  exactly  the  same  rate 
from  week  to  week,  they  run  closely  enough  for  the  practical  purposes  of  navigation. 


IO6  DESCRIPTIVE    ASTRONOMY. 

~~(^)    What  is  the  north  polar  distance  of  a  star  whose  declination 
is  -30°  43'? 

-(c)    What  is  the  right  ascension  of  the  autumnal  equinox? 
^d)   WThat  is  the  sun's  right  ascension,  when  it  is  in  the  summer 
solstice? 

j(^)    What  is  its  right  ascension  when  in  the  winter  solstice? 
—  (f)    If  the  vernal  equinox  be  on  the  meridian  of  Chicago,  what 
is  the  right  ascension  of  a  star  which  is  rising  at  that  instant  at  the 
east  point  of  the  Chicago  horizon? 

"(«£")    At  the  same  instant  as  above  a  star  is  setting  at  the  west 
point  of  the  horizon;   what  is  its  right  ascension? 
—  8.    (a)  If  a  star  having  a  right  ascension  of  .18  hours  is  now  on 
the  meridian,  what  is  the  right  ascension  of  a  star  which  now  has  an 
east  hour  angle  of  3  hours? 

^.  (b)  When  a  star  the  right  ascension  of  which  is  8  hours  is  on 
the  meridian,  what  is  the  right  ascension  of  a  star  which  has  a  west 
hour  angle  of  5  hours? 

(c)    May    a    number    of    different    stars    have    the    same    right 
ascension? 
— (d)    May  a  number  of  stars  have  the  same  declination? 

(e)    Is  it  possible  for  two  stars  to  lie  on  the  same  hour  circle,  and 

have  different  right  ascensions? 

«r  (/)    If   the  right  ascension   of  a  fixed  star  now  is   1 1  h.  28  m., 
what  is  it  3  hours  hence? 

^-  9.    (a)    When  a  correct  sidereal  clock  reads  17  h.,  the  hour  angle 
of  a  certain  star  is  5  h.  west.     What  is  the  star's  right  ascension  ? 
•*£  (£)    The  sidereal  time  being   17  h.  26m.,  a  star  is  found  to  have 
an  east  hour  angle  of  4h. ;  what  is  the  star's  right  ascension? 

10.    Reduce  March  9,  7  A.  M.,  civil  time,  to  astronomical  time. 
^-u.    The  sun  and  a  star  are  in  the  vernal  equinox,  and  both   are 
setting  below  the  horizon  of  some  place. 
-  (#)    A  month  afterwards,  which  will  set  the  earlier? 
\b)    Which  will  rise  the  earlier? 

'  \c)    On  July  2Oth,  a  certain  star  sets  below  the  horizon  of  Boston 
at  8  P.  M.     A  month  afterwards,  will  it  set  at  about  the  same  time? 

12.  By  use  of  the  telegraph  it  was  found  that  when  a  Washing- 
ton clock  read  9h.  o  m.  o  sec.,  a  St.  Louis  clock  read  8  h.  7m. 
22.93  sec.  The  Washington  clock  was  26.37  sec.  fast>  and  the  St. 


CELESTIAL    MEASUREMENTS.  IO7 

Louis  clock  was  15. 92  sec.  slow.     What  is   the  difference  of  longi- 
tude between  the  two  places? 

13.  An  astronomer  noted  the  following  readings  of  his  sidereal 
clock  when  a  certain  star  crossed  the  wires  in  the  reticle  of  his 
meridian  circle. 

h.                     111.  sec. 

13                   14  iy.2 

14  30.6 

14  44.0 

14  57-4 

15  10.7 

The  right  ascension  of  the  star  was  I3h.  1401.  3 1.68  sec.  If 
the  meridian  circle  was  in  perfect  adjustment,  what  was  the  clock 
error? 

—14.  When  Polaris  is  on  the  meridian  above  the  pole,  an  observer 
measures  its  altitude,  finding  it  to  be  39°  46'.  Twelve  hours  later 
its  altitude  is  37°  18'.  What  is  the  latitude  of  the  place  of 
observation  ? 

— 15.  (a)  If  the  altitude  of  a  star  is  16°,  what  is  its  zenith 
distance? 

rr  (£)    If  the  declination  of  a  star  is  +34°  5',  what  is  its  north  polar 
distance? 

^.Ajc)    The  north  polar  distance  of  a  star  is  116°  35'.     What  is  its 
declination? 

1 6.  A  mariner  measures  the  altitude  of  the  sun  at  noon,  getting 
69°  47'  25"  as    his    result.      From  the  almanac   he    finds  that  the 
sun's  declination  at  the  time  when  the  altitude  was  measured  was 
—6°  37'  49".     Find  the  latitude  of  the  ship's  position. 

17.  At   midnight  on   July  25th,  a   chronometer  was    38.92  sec. 
slow.     Its  daily  rate  was  0.84  sec.  gaining.     Assuming  that  it  kept 
this  rate,  what  was  its  error  at  noon  of  July  3 1st? 

1 8.  The  captain  of  a  vessel  measures  the  altitude  of  the  sun  on 
the  afternoon  of  May  i6th;  his  chronometer  (keeping  Greenwich 
time)  reads  4  h.  i6m.  29  sec.  when  the  altitude  is   measured.     By 
means  of  data   given  in  the  almanac,  he  computes  that  the   local 
time  when  the  sun  had  the  measured  altitude  was  3  h.  27m.  58  sec. 
What  was  the  longitude  of  the  ship  at  the  time  of  the  observation  ? 


108  DESCRIPTIVE    ASTRONOMY. 


CHAPTER   VII. 

THE   MOON   AND   ECLIPSES. 

"  In  full-orbed  glory  yonder  moon  divine 
Rolls  through  the  dark  blue  depths." 

SOUTHEY. 

145.  Distance,  Diameter,  Orbit,  Nodes. — The  moon,  though  the 
most  conspicuous  of  all  the  heavenly  bodies  except  the  sun,  is 
really  quite  small,  being  only  2,163  miles  in  diameter. 

Its  average  distance  from  the  earth's  centre  is  238,840  miles.  As 
the  earth  journeys  around  the  sun  in  an  ellipse,  so  the  moon  travels 
around  the  earth  in  a  similar  orbit,  the  earth  being  at  one  of  the 
foci  of  the  ellipse.  The  orbit  is  nearly  a  circle,  and  is  inclined  to 
the  plane  of  the  ecliptic  only  5°.  The  moon  appears  to  us  to  de- 
scribe a  circle  on  the  face  of 
the  sky  every  month,  the  circle 
being  roughly  coincident  with 
the  ecliptic.  The  moon's  path 
intersects  the  ecliptic  at  two 
opposite  points,  called  the 
moon's  nodes. 

146.    Periods,    Sidereal    and 

Fi?.  100.  —  ORBITS  OF  THE  EARTH  AND  MOON.     «         •,.  -r^,          •  7          /    ,       •     » 

Synodic.  —  The  sidereal  period 

of  the  moon  is  the  time  required  for  making  one  revolution  about 
the  earth.  In  Fig.  101  the  sun,  the  earth,  and  the  moon  are  in  line, 
in  the  positions  S,  E,  and  M.  The  earth  and  the  moon  pursue  their 
appointed  paths  until  they  arrive  at  E'  and  M'  respectively,  the  line 
E'M'  being  parallel  to  EM.  The  moon  has  now  accomplished  a 
complete  revolution ;  the  time  required  is  nearly  2/|  days,  which  is 
therefore  the  sidereal  period.  But  the  moon  is  not  yet  opposite  the 
sun,  as  it  was  at  the  start.  When  it  reaches  M",  the  earth  being  at 
E",  it  is  opposite  the  sun  again,  and  a  synodic  revolution  has  been 
accomplished.  The  synodic  period 'is  over  29!  days. 


THE    MOON    AND    ECLIPSES. 


IO9 


147.  Time  of  Crossing  the  Meridian.  —  Since  the  moon  moves  rap- 
idly eastward  among  the  stars,  it  does  not  cross  the  meridian  of 
the  observer  at  the  same  time  every  day,  but  crosses  about  5 1  min- 
utes later  on  the  average  each  day  than  the  preceding.  The  amount 
of  daily  retardation  varies  considerably  from  causes  analogous  to 
those  which  cause  the  sun  to  be  an  irregular  timekeeper.  (§  126.) 


A; 


Fig.  101.  —  SIDEREAL  AND  SYNODIC  PERIODS. 


Fig.  102.  —  ILLUSTRATION  OF  THE 
MOON'S  ROTATION. 


148.  Rotation.  —  The  moon  always  presents  the  same  face  to  us : 
an  unthinking  person  might  conclude  from  this  that  it  did  not  rotate 
at  all.     Let  a  boy  trace  a  circle  on  the  ground  around   a  tree,  and 
station  himself  south  of  the  tree  and  facing  it ;   he  then  faces  north. 
Let  him  walk  around  the  circle,  continually  facing  the  tree.     At  S 
in  Fig.  1 02,  he  faces  north,  at  E  west,  at  N  south,  and  at  W  east. 
When  he  arrives  at  S  again,  he  has  turned  completely  around  once. 

149.  Librations. — The  moon  rotates  on  its  axis  at  a  uniform  rate, 
but  since  it  does  not  move  with  uniform  rapidity  in  its  orbit,  we 
sometimes  see  a  short  distance  around  one  edge  or  the  other.     In 
Fig.  103,  when  the  moon  is  at  M,  we  see  the  portion  ACB.     When 
moving  more   swiftly   than  its   average,  it  will   describe  90°  of  its 
orbit  and  arrive  at  M'  in  a  little  less  than  one  fourth  of  its  sidereal 
period.     So  it  will  not  have  rotated  one  fourth  of  a  complete  turn, 
and  an  observer  at  the  earth,  though  not  able  to  see  the  point  A, 
will  look  past  B,  as  shown  by  the  figure. 


no 


DESCRIPTIVE    ASTRONOMY. 


Furthermore,  the  moon  does  not  stand  quite  upright  on  its  orbit, 
that  is,  its  axis  is  not  perpendicular  to  the  plane  of  its  orbit.  Hence, 
as  shown  in  Fig.  104,  we  sometimes  see  past  the  north  pole  and 
sometimes  past  the  south  pole. 

These  apparent  oscillations  are  called  librarians.  There  is  a 
minute  libration  due  to  the  fact  that  we  are  not  at  the  earth's  centre. 


N 


EARTH 


Fig.  103.  —  LIBRATION. 


Fig.  104.  —  LIBRATION. 


When  the  moon  is  rising,  we  can  see  farther  over  its  upper  edge 
than  a  man  whose  eye  is  at  the  earth's  centre.  Fifty-nine  per  cent 
of  the  moon's  surface  has  been  seen  by  astronomers. 

150.  Phases  of  the  Moon.  —  The  moon  shines  by  reflecting  the 
sunlight  which  strikes  it.  When  the  moon  is  between  us  and  the 
sun,  its  illuminated  side  being  toward  the  sun,  we  see  the  dark  side. 
The  moon  is  then  said  to  be  new.  A  week  later,  when  it  has  moved 
from  A  to  B  (Fig.  105),  half  of  the  illuminated  hemisphere  is  visible 
to  us,  and  the  moon  is  said  to  be  at  its  first  quarter.  After  the 
lapse  of  another  week  it  is  at  C,  opposite  the  sun,  and  the  whole  of 
the  illuminated  hemisphere  can  be  seen  by  us ;  this  phase  is  full 
moon.  A  week  thereafter,  when  the  moon  has  reached  D,  it  is  in 
its  last  quarter,  half  the  bright  hemisphere  being  visible.  For  a 
week  before  and  after  new  moon,  when  but  a  small  part  of  the  moon 
looks  bright  to  us,  it  is  crescent.  During  the  week  preceding  full 
moon,  and  also  during  the  following  week,  when  more  than  one  half 
and  less  than  the  whole  of  the  bright  part  of  the  moon  is  turned 


THE    MOON    AND    ECLIPSES. 


I  II 


toward  us,  the  moon  is  gibbons.     The  appearances  of  the  moon  are 
shown  in  Fig.  106. 

151.  Earth  Shine.  — When  the 
moon  is  crescent,  one  easily  sees 
the  dark  portion  of  it,  as  well  as 
the    brilliant   crescent   of  light. 
The  dark  part  is  bounded  on  the 
side  next  to  the  sky  by  a  narrow 
rim  of  silvery  light,  so  that  the 
whole  looks   not    unlike  a  cake 
basket  hung  in    the    sky :    it  is 
popularly  called  "  the  old  moon 
in  the  new  moon's  arms."     Why 
is  the  dark  part  seen  so  easily? 
Some  of  the  sunlight  which  falls 
upon  the  earth  is  reflected  away, 
and,  striking  upon  the  side  of  the 
moon  turned  toward  us,  illumi- 
nates it  sufficiently  to  render  the 
dark  part  visible  to  us. 

152.  Occupations. — The  moon 
in  its  monthly  round  passes  be- 
tween us  and  many  of  the  stars,  hiding  them  from  view  so  that  they 
are  occulted  (hidden)   for  a  time.     The  occultation  of  bright  stars 


105.  —  THE  MOON  ILLUMINATED. 


NEW 


CRESCENT 
WAXING 


FIRST 
QUARTER 


FULL 


GIBBOU5 
WANING 


LA5T 
QUARTER 


GIBBOUS 
WAXING 


CRESCENT 
WANING 


Fig.  106. — THE  MOON'S  PHASES. 

can  be  observed  with  the  naked  eye.     The  fainter  ones  are  blotted 


112  DESCRIPTIVE     ASTRONOMY. 

out  by  the  moon's  brilliancy  as  it  approaches  them.  At  the  instant 
when  the  limb  of  the  moon  gets  into  line  between  the  star  and  the 
observer's  eye,  the  star  vanishes  as  if  annihilated.  After  an  hour  or 
less,  the  star  reappears  on  the  opposite  side  of  the  moon.  Both  the 
disappearance  and  the  reappearance  are  instantaneous.  Observa- 
tions of  occultations  are  sometimes  made  by  mariners  to  find  the 


Fig.  107. — THE  MOON:    PHOTOGRAPHED  AT  THE  LICK  OBSERVATORY. 

errors  of  their  chronometers :  from  the  data  given  in  the  Nautical 
Almanac  the  Greenwich  time  of  the  occurrence  of  an  occultation  of 
a  given  star,  as  seen  at  a  given  place,  is  computed.  The  chronom- 
eter reading  is  noted  at  the  time  of  the  star's  disappearance ;  by 
comparing  this  reading  with  the  computed  time  the  error  of  the 
timepiece  is  found. 


UNIVERSITY 
THE    MOON    AND    ECLIP^Sof  0=0^^     I  13 


153.  Appearance  to  the  Naked  Eye.  —  To  the  naked  eye,  the  face 
of  the  full  moon  appears  to  be  diversified  with  irregularly  shaped 
dark  spots.     Most  people  see   a  strong  resemblance  to   a  human 
face.     Many  perceive  a  complete    human    figure,  said    among  the 
French  to  be  Judas  Iscariot,  transported  thither  as  a  punishment. 
Humboldt  states  that  it  is  a  popular   belief  among  the   people  of 
Asia    Minor   that   the    moon  is  a  mirror,  which  reflects  back  the 
image  of  the  earth.     When  one  examines  the  moon  with  an  opera- 
glass,  the  dark  spots  are  seen  to  be  the  smoother  portions  of  the 
moon's  surface.     They  are    simply  vast  plains :   on  the  maps  they 
are  designated  as  seas,  being  thought  by  the  early  lunar  cartogra- 
phers to  be  such. 

154.  Use  of  the  Telescope.  —  It  is  well   at  first  to  put  on  a  low 
magnifying  power,  so   that  the  whole  of  the  moon  may  be  in  the 
field  of  view  at  once.     At  the  time  of  full  moon  the  view  is  very 
much  less  satisfactory  than  at  the  first  quarter  (Fig.  107),  and  for 
three  days  thereafter.      For  at  the  time  of  full  moon  the  shadows 
of  the    mountains   on  the  moon  are   invisible  to  us,  because  they 
are  cast  directly  behind  them,  and  we  lose  the  effect  of  contrast 
between  the  objects  and   their  shadows.     At  the  time  of  the  first 
quarter,  those  mountains  which  are  near  the  terminator  (boundary 
between  the  illuminated  and  unilluminated   portions  of  the  moon) 
cast  magnificent  shadows ;   the  rugged  details  of  these  mountainous 
forms    are    then    very   conspicuous    in    the    telescope    (Fig.    113). 
After   viewing    the    entire    lunar    disk   with    a   low   power,    higher 
powers    may  be    tried    with   advantage    on  the    more    conspicuous 
objects.     They  bring  out  a  wealth  of  detail,  which  wellnigh  baffles 
delineation. 

No  telescope  ever  yet  constructed  can  bear  with  advantage,  even 
on  the  finest  night,  a  power  exceeding  3,000  diameters :  such  a 
power  would  bring  the  moon  within  80  miles. 

Objects  as  large  as  the  largest  buildings  on  the  earth  might  be 
perceived,  if  they  differed  considerably  in  color  from  the  back- 
ground upon  which  they  were  seen. 

155.  Lunar  Topography.  —  The  features  of  the  landscape  maybe 
divided  into  the  following  classes :   plains,  craters,  mountain  peaks, 
mountain  ranges,  rills,  clefts,  and  rays.     Some  hundreds  of  these 
objects  have  received  names :   a  few  of  the  most  prominent  ones 


DESCRIPTIVE    ASTRONOMY. 


are  shown  on  the  accompanying  skeleton  map  (Fig.  108),  in  which 
the  moon  is  represented  as  seen  in  an  inverting  telescope.  The 
numbered  craters  have  the  following  names :  — 


1.  Clavius. 

2.  Schiller. 

3.  Schickard. 

4.  Tycho. 

5.  Catharina. 


6.  Cyrillus. 
7.  Theophilus. 
8.  Arzachael. 
9.  Alphonsus. 
10.  Ptolemy. 

ii. 

12. 

13- 
14. 

1S- 

Gassendi. 
Maskelyne. 
Copernicus. 
Kepler. 
Eratosthenes. 

/^'o 

& 

q 

X  -N 

D7^ 

,--.          fV      C 

1  6.  Archimedes. 

17.  Burg. 

1  8.  Aristotle. 

19.  Plato. 


Fig.  108.  —  SKELETON  MAP  OF  THE  MOON. 

156.  The  Plains.  —  These  are,  as  before  remarked,  darker  than 
the  rest  of  the  surface,  and  smoother.  With  low  powers  they  look 
much  as  if  they  were  dry  beds  of  ancient  seas.  Under  a  high 
power  many  minute  pits  are  discovered  besprinkling  the  plains ; 


THE    MOON    AND    ECLIPSES.  115 

the  surface  is  found  to  be  really  quite  rough  and  wrinkled.  The 
boundaries  of  these  "  seas,"  as  they  are  denominated  on  the  map, 
are  not  always  sharply  defined ;  in  some  cases,  the  bounding 
"  sea-wall "  is  nearly  complete,  and  composed  in  part  of  pre- 
cipitous cliffs,  which  exceed  in  grandeur  any  similar  terrestrial 
formations. 


Fig.  109.  —  CONSPICUOUS  CRATERS.     (Nasmyth  and  Carpenter.) 

157.  Craters.  —  Even  with  an  opera-glass  one  may  see  that  the 
brighter  portions  of  the  moon's  surface  are  thickly  bestrewn  with 
irregular  ring-shaped  mountains.  Kepler1  conjectured  that  these 

1  The  great  astronomer,  1571-1630,  who  discovered  that  the  orbits  of  the.  planets 
are  ellipses,  and  formulated  certain  famous  laws  concerning  their  motion. 


n6 


DESCRIPTIVE    ASTRONOMY. 


were  pits  dug  by  the  inhabitants  of  the  moon  to  shelter  themselves 
from  the  sun  during  the  long  lunar  day. 

Had  he  known  that  a  large  number  of  these  pits  are  over  fifty 
miles  in  diameter,  and  that  the  walls  of  some  of  them  are  three  or 
four  miles  high,  he  would  hardly  have  broached  this  theory.  The 
craters  present  a  wonderful  diversity  of  size  and  aspect.  Thou- 


•  ••''.-; 
*•&»' 


%fe 


|il|»^il 

lil«ll 


_ 

Fig.  no.  —  THE  CRATER  COPERNICUS.     (Nasmyth  and  Carpenter.) 

sands  are  so  minute  that  in  the  most  powerful  telescope  they  look 
like  mere  pin-pricks,  being  half  a  mile  or  less  in  diameter.  The 
largest  ones  are  over  one  hundred  miles  across,  and  are  perhaps 
more  properly  called  walled  plains,  especially  if  the  floor  of  the 
crater  is  quite  smooth.  In  the  centre  of  a  crater  a  mountain  or 
a  small  group  of  peaks  is  usually  found. 


THE    MOON    AND    ECLIPSES.  117 

The  interiors  of  most  of  these  craters  are  lower  than  the  general 
surrounding  level,  but  in  some  cases  the  interiors  are  elevated 
above  the  general  surface.  Some  are  isolated :  others  are  crowded 
so  thickly  together  that  they  overlap.  Some  walls  are  very  pre- 
cipitous ;  others  are  a  series  of  magnificent  terraces.  The  im- 
mensity of  some  of  these  formations  is  realized  by  looking  at  the 
cut  of  Copernicus  (Fig.  no).  The  larger  of  the  craterlets  around 
it  are  as  big  as  Vesuvius.  In  the  figure,  these,  as  well  as  the  other 
features  to  be  described,  are  easily  distinguished. 


Fig.  in.  —  COPERNICUS. 

158.  Nature  and  Cause  of  the  Craters.  —  The  craters  are  frequently 
referred  to  as  the  moon's  volcanoes.  It  would  be  a  mistake  to 
infer  from  this  that  there  are  any  signs  of  present  volcanic  activity 
on  the  moon.  The  entire  appearance  points  to  the  theory  that 
the  moon  was  once  a  molten  mass,  and  that  by  its  cooling  and 
solidification  its  various  topographical  features  were  formed. 

Similar  appearances  are  to  be  found  upon  the  earth;  Fig.  112 
shows  the  marked  similarity  of  the  neighborhood  of  Vesuvius  to 
a  portion  of  the  moon.  On  the  cooling  tap  cinder  from  the  fur- 
naces for  the  production  of  iron  there  is  formed  a  thin  crust, 
which  is  soon  broken  open  in  spots  by  the  pressure  of  the  con- 


n8 


DESCRIPTIVE    ASTRONOMY. 


fined  gases ;  the  molten  matter  exudes  through  the  holes  and 
cracks,  and  frequently  forms  miniature  volcanic  cones.  The  cen- 
tral mountains  in  lunar  craters  were  probably  formed  by  the  last 
sluggish  oozings  from  the  heated  interior. 

159.  Mountains  and  Mountain  Ranges.  —  Isolated  mountain  peaks 
are  comparatively  rare,  though  occasionally  one  can  be  found 
rising  to  a  height  of  a  mile  or  more  out  of  a  comparatively  smooth 
landscape.  There  are  a  few  mountain  chains,  the  most  prominent 


Fig.  112. — THE  TERRESTRIAL  CRATER  VESUVIUS.     (Nasmyth  and  Carpenter.) 

of  which  is  named  the  Apennines ;  it  is  450  miles  long,  and  bristles 
with  peaks  which  rival  the  Andes  in  altitude,  and  cast  magnificent 
shadows  nearly  100  miles  long  athwart  the  neighboring  plains. 
By  measurement  of  these  shadows  the  heights  of  many  peaks  have 
been  determined.  On  one  side,  as  shown  in  Fig.  113,  they  rise 
gradually  from  the  plain,  but  on  the  other  they  are  terminated  by 
dizzy  precipices,  some  of  which  are  over  three  miles  high. 

160.    Rills,  Clefts,  and  Rays.  —  Rills    are    narrow,  deep,  and  tor- 
tuous valleys,  which  look  like  the  beds  of  dried  up  streams. 


THE    MOON    AND    ECLIPSES.  119 

Clefts  are  narrow  rifts  of  great  depth.  Two  fine  ones  are  shown 
in  Fig.  113,  starting  from  opposite  sides  of  the  largest  crater:  each 
is  over  100  miles  in  length ;  near  the  centre  it  is  a  mile  in  width. 
They  are  thought  to  be  not  less  than  ten  miles  in  depth,  and 
must  be  appalling  in  grandeur  to  a  lunar  traveller. 


Fig.  113.  —  THE  LUNAR  APENNINES.     (Nasmyth  and  Carpenter.) 

Rays  are  streaks  which  diverge  in  all  directions  from  some  of 
the  craters.  They  are  best  seen  at  the  time  of  full  moon.  The 
finest  system  radiates  from  Tycho,  which,  with  an  opera-glass, 
looks  as  if  it  were  a  pole  of  the  moon,  the  rays  being  meridians 
diverging  from  it.  Copernicus  has  a  smaller  system,  shown  in  Fig. 
109.  The  rays  are  on  the  general  surface,  being  neither  elevated 


120 


DESCRIPTIVE    ASTRONOMY. 


above  nor  depressed  below  it;  they  go  over  crater  walls  and 
through  valleys,  just  as  if  some  one  had  painted  them  there  with 
a  gigantic  brush  after  the  landscape  had  assumed  its  present  form. 
No  satisfactory  explanation  of  these  has  been  given.  Possibly 


Fig.  114.  —  THE  CRATER  VENDELINUS  :   DRAWN  FROM  A  LICK  PHOTOGRAPH. 

they  are  discolorations  of  the  surface  by  vapors  rising  from  cracks 
too  narrow  to  be  visible  to  us. 

161.    Changes. — There  has   been   considerable  discussion  among 
astronomers  as  to  whether  any  changes  have  ever  been  noted  in  the 


THE    MOON    AND    ECLIPSES.  121 

lunar  topography.  A  given  crater  may  change  its  aspect  in  an 
hour,  because  of  the  shifting  of  its  shadow ;  such  changes  are  most 
noticeable  when  the  sun  is  just  rising  or  setting  at  the  crater  (Fig. 
114).  For  this  reason,  a  careful  examination,  extending  over 
several  nights,  is  necessary  to  enable  one  to  gain  a  correct  idea  of 
the  details  of  the  form  of  any  crater  or  mountain.  When  we  add 
to  this  the  usual  blurring  caused  by  the  unsteadiness  of  our  atmos- 
phere, and  the  minute  errors  which  the  most  skilful  draughtsmen 
are  liable  to  make  in  their  delineations,  we  can  see  how  easy  it  is 
to  imagine  slight  changes  where  none  have  really  taken  place. 
There  is  not  the  slightest  evidence  that  any  eruptive  forces  are  at 
work.  Possibly  a  very  few  land-slips  have  occurred.  Lunar  pho- 
tography may  after  some  years  give  us  decisive  results. 

162.  Atmosphere.  —  It  has  been  demonstrated  that  the  atmosphere, 
if  it  exists,  is   extremely  rare,  the  pressure  not  exceeding  a  thou- 
sandth of  that  at  the  earth's  surface.     When  a  star  is  occulted,  it 
ought,  if  there  were  a  lunar  atmosphere  one  tenth  as  dense  as  that 
of  the  earth,  to  suffer  a  change  of  brightness  and  color,  when  close 
to  the  moon's  limb ;    further,  as  one  sees  the  sun  after  it  has  really 
set,  on  account  of  refraction,  so  the  time  of  the  star's  disappearance 
would  be  much  retarded  by  the  refraction  of  the  lunar  atmosphere, 
and  its  reappearance  would  be  accelerated. 

Twilight  causes  an  illumination  of  the  terrestrial  landscape  for 
some  time  after  the  sun  has  set.  No  marked  illumination  of  this 
sort  has  been  seen  at  any  point  on  the  moon. 

The  lunar  spectrum  is  identical  with  the  solar ;  this  shows  that 
the  sun's  rays  when  reflected  from  the  moon  suffer  no  noticeable 
absorption  by  its  atmosphere. 

163.  Water.  — What  may  be  on  the  side  of  the  moon  which  we 
never    see,  we    cannot    affirm,  but  there    is   no   reason   to   think    it 
different  from  the  face  presented  to  us.     Any  lake  covering  as  much 
as  a  square   mile,   if  not  hidden  from  view  by  some   obstruction, 
would  have  been  discovered  ere  this. 

On  account  of  the  coldness  of  the  lunar  days,  as  well  as  nights,  it 
is  not  unlikely  that  water,  if  present,  would  exist  only  in  a  frozen 
state.  There  are  no  indications  of  either  ice  or  snow. 

164.  The  Water  and  Air  Formerly.  —  If  the  moon  was  once,  as   is 
generally  supposed,  a    portion  of  the  earth,   it  must  have  carried 


122  DESCRIPTIVE    ASTRONOMY. 

some  water  and  air  with  it  when  they  separated,  though  the  earth 
kept  the  lion's  share,  on  account  of  its  stronger  power  of  attraction. 
What  has  become  of  the  water  we  can  only  conjecture :  great  cav- 
erns may  have  been  formed  in  the  process  of  cooling,  into  which 
both  the  air  and  water  have  sunk.  The  water  may  have  become 
chemically  united  with  the  molten  rock  in  the  process  of  crystal- 
lization. A  rock  when  heated  expels  gases  formerly  absorbed ; 
in  cooling  slowly  it  can  absorb  them  again ;  perchance  the  lunar 
atmosphere  was  absorbed  in  this  way.  Still  another  theory  is 
based  on  the  exceedingly  swift  motion  of  the  molecules  of  gases. 
The  force  of  gravity  at  the  moon's  surface  is  but  one  sixth  of  that 
at  the  earth's,  so  that  a  rifle  bullet  there  would  "  carry"  100  miles. 
Some  have  thought  that  the  molecules  of  the  lunar  atmosphere 
may  have  escaped  from  this  feeble  attraction  and  gone  off  into 
space,  never  to  be  recovered ;  but  this  theory  will  not  stand  a  critical 
examination. 

165.  Light  and  Heat  reflected  to  the  Earth.  —  Five    sixths   of  the 
sunlight   which    falls  upon  the  moon   is    absorbed,  the   rest  being 
reflected  away.     The  sun  gives  600,000  times  as  much  light  as  the 
full  moon ;   yet  the  full  moon  in  mid-heaven  gives  sufficient  light  to 
enable  one  to  read  this  page.     The  measurement  of  the  heat  sent 
us  by  the   moon  is  difficult,  because    its    amount   is    "  vanishingly 
small."     If  the  full  moon  could  shine  upon  us  steadily  for  a  year, 
it  would  give  us  as  much  heat  as  the  sun  does  in  three  minutes. 

166.  Temperature  at  the  Moon.  —  During  the  long  lunar  day  the 
sun  blazes  upon  the   moon's  plains  with  a   fury  unmitigated  by  a 
protecting  atmosphere,  and  untempered  by  the  presence  of  clouds ; 
yet  the  plains  are  cold.     The  air  of  our  own  planet  acts  as  a  blanket 
to  keep  us  warm.     The  solar  rays  pierce  the    atmosphere   readily 
and  find  lodgment  in  the  earth ;   but  when  the  earth  strives  to  radi- 
ate its  heat  back  into  space,  the  air  checks  the  radiation.     On  lofty 
mountain  tops,  over  which  there  is  a  much  thinner  air  blanket  than  at 
their  base,  the  rigors  of  eternal  winter  reign.     The  lunar  atmosphere 
is  entirely  inadequate  to  check  radiation,  so  that  under  direct  sun- 
shine  the  temperature  of  the  moon's  surface  probably  never  rises 
above  the  freezing  point  of.  water.     During  the  lunar  night  the  tem- 
perature is  believed  to  be  no  higher  than  200°  below  zero  on  the 
Fahrenheit  scale. 


THE    MOON    AND    ECLIPSES.  123 

167.  Life  on  the  Moon.  —  Enough  has  been  said  to  show  that  there 
is  no  such  animal  and  vegetable  life  on  the  moon  as  on   the   earth. 
It  is  a  land  of  death.      The   sky  is   a   pall    of  black,  studded  with 
stars  by  day  as  well  as  by  night.     The  rising  sun,  unheralded  by  the 
beautiful  sky  tints  which  accompany  the  dawn   on  earth,  darts  his 
garish  beams  athwart  the  desolate  landscape,  causing  the  lofty  peaks 
to  cast  long  shadows  which  vie  with  the  sky  in  blackness.     No  bird 
song  greets  him;  there  is  no  rustle  of  a  breeze,  or  plash  of  a  brook, 
or  murmur  of  an  ocean.     Should  "  lips  quiver  and  tongues  essay  to 
speak,"  no  sound  from  them  would  break  the  eternal  silence.     Dark 
pits  innumerable  yawn  on  every  hand.     The  silvery  rims  of  mighty 
craters  encircle  abysses  of  darkness.     As  the  sun  slowly  rises  in  the 
sky,  the  fierce  chill  of  the  departing  night  is  slowly  mitigated ;   but 
no  manlike  being  welcomes  returning  warmth. 

The  earth  hangs  continually  in  mid-heaven,  waxing  from  crescent 
to  full  and  waning  again,  swiftly  spinning  on  its  axis  and  bringing 
into  view  an  ever  shifting  panorama  of  cloud  and  continent  and 
ocean.  No  star  forgets  to  shine ;  the  weird  glory  of  the  solar 
corona  and  the  fantastic  forms  of  the  protuberances  can  be  seen  in 
all  their  beauty  by  screening  off  the  direct  light  of  the  sun.  The 
Milky  Way  girdles  the  sky,  bejewelled  with  thousands  of  glittering 
orbs.  The  eye  is  enchanted  by  the  glories  above,  though  the  mind 
shrinks  from  contemplation  of  the  desolation  all  about.  After  four- 
teen terrestrial  days  have  elapsed,  the  long  shadows  stretch  them- 
selves eastward,  the  sun  slowly  sinks  beneath  the  western  horizon, 
and  night  with  its  terrible  rigors  of  cold  comes  on  apace.  Such  is  a 
lunar  day. 

168.  The  Moon  and  the  Weather.  —  Various  fanciful   notions  con- 
cerning the  moon's  influence  upon  the  weather  are  rife  among  igno- 
rant persons.     One  hears  of  wet  and  dry  moons ;   when  the  cusps 
of  the  new  moon  have  a  decided  upward  slant,  fair  weather  is  said 
to  be  presaged;  when  they  do  not    slant  upwards,  foul  weather   is 
to  be  expected.     Such  ideas   are  arrant   nonsense.     The   positions 
of  the  moon's  cusps  can  be  foretold   for  thousands  of  years ;   the 
weather,  not  for  a  single  week.     The  connection  of  changes  of  the 
weather  with  changes  of  the  moon's  phases  is  likewise  unfounded. 
Since  the  moon  changes  its  phase  every  week,  all  weather  changes 
must  occur  within  four  days  of  some  change  of  lunar  phase.     We 


124  DESCRIPTIVE    ASTRONOMY. 

know  of  no  ways  in  which  the  moon  would  affect  the  weather  except 
by  its  heat,  or  by  raising  aerial  tides,  or  by  disturbances  of  the 
magnetic  conditions. 

Its  heat  is  almost  immeasurably  small ;  the  effect  of  aerial  tides 
on  the  readings  of  the  barometer  is  insignificant.  Certain  minute 
magnetic  disturbances  have  been  detected,  which  seem  to  be  de- 
pendent upon  the  varying  distance  of  the  moon.  The  idea  that  the 
full  moon  clears  away  clouds  probably  has  its  foundation  in  the  fact 
that  the  moon  renders  the  rifts  in  a  lightly  clouded  sky  conspicuous, 
while  they  would  otherwise  escape  notice. 

169.  The  Moon's  Worth  to  Man.  —  The  most  stupendous  work  done 
by  the  moon  for  man  is  the  rise  of  the  tides,  of  which  it  is  the  chief 
cause.     The  flood  tide  lifts  ponderous  ships  over  dangerous  bars  at 
the  entrances  of  harbors.    Merchantmen  are  carried  from  the  mouth 
of  the  Thames  up  the  river  to  the  busy  wharves  of  London  on  the 
bosom  of  the  tide.     The  tides  scour  the  mouths  of  rivers,  carrying 
away   the    pestilence  breeding  matter  which   tends  to   accumulate 
there. 

The  enormous  power  of  the  tides  may  some  day  be  utilized  in 
driving  dynamos  to  charge  storage  batteries,  from  which  electricity 
can  be  taken  when  desired. 

The  moon  also  helps  the  navigator  to  guide  his  ship,  as  explained 
in  §  152.  In  this  capacity  the  moon  has  frequently  been  likened  to 
the  hand  of  a  stupendous  clock,  whose  dial  is  the  starry  vault. 

In  historical  researches  dates  are  frequently  fixed  by  reference 
to  eclipses,  which  inspired  awe  in  the  beholders,  and  were  carefully 
recorded.  The  date  of  the  beginning  of  the  Christian  era  is  deter- 
mined by  means  of  a  lunar  eclipse,  which  took  place  on  the  night 
of  Herod's  death.  The  moon's  light  is  of  use  in  various  ways, 
which  readily  suggest  themselves. 

ECLIPSES. 

170.  How  Caused :  Shape  of  the  Shadow.  —  An  eclipse  of  the  moon 
occurs  when  it  is   in  the   shadow  of  the  earth ;   one  of  the  sun   is 
caused  by  the  interposition  of  the  moon   between  it  and  us.     In 
order  to  understand  them,  we  must  study  the  shape  of  the  shadow 
cast  by  the  earth  or  moon. 


THE    MOON    AND    ECLIPSES. 


125 


In    Fig.    115,  S,  E,  and  M   represent  the  sun,  earth,  and  moon, 

respectively.     The  heavily  shaded  portion  CDV  is  called  the  umbra 

of  the  earth's  shadow;   it  is  of 

a    conical    shape.      The  lightly 

shaded  portions,  FCV  and  GDV, 

represent   the  penumbra  of  the 

shadow.     An  eye  situated  at  X, 

between  CF  and  CV,  would  see 

(neglecting  the  effect  of  refrac- 
tion of  the  sun's  rays,  where  they 

graze    the    earth    at  C)   only   a 

portion  of  the  sun's   disk.     An 

object  between  CF  and  CV  would 

not  be  as  brilliantly  illuminated 

as  one  at  the  left  of  CF,  where 

light  from  every  part  of  the  sun's 

disk  would  strike  it. 

In   Fig.    116,   CHKD   is   the 

umbra  of  the  moon's    shadow; 

the  penumbra  occupies  the  space 

represented  by  FCH  and  KDG. 

A  cross-section  of  the  shadow 

of  either  the  earth  or  the  moon 

is  shown  in  Fig.   117,  the  dark 

portion    being   the    umbra,    the 

lighter  the  penumbra. 

171.  Cause  of  a  Lunar  Eclipse.  —  Since  the  centres  of  the  sun  and 
earth  lie  in  the  plane  of  the  ecliptic,  the  axis 
of  the  earth's  shadow,  a  line  drawn  from  E 
to  V  in  Fig.  115,  lies  there  also.  If  the 
moon  moved  exactly  in  the  plane  of  the 
ecliptic,  it  would  pass  through  the  earth's 
shadow  every  month,  and  suffer  eclipse.  But 
as  the  moon  is  above  or  below  the  ecliptic, 
except  when  at  its  nodes  (§  145)  it  usually 
passes  above  or  below  the  earth's  shadow 
and  escapes  eclipse.  On  those  occasions 

when  it  encounters  the   shadow,  the  eclipse   is  total   if  the  entire 


Fig.  115.  —  UMBRA 
AND  PENUMBRA 
OF  THE  EARTH'S 
SHADOW. 


Fig.  116.  —  UMBRA 
AND  PENUMBRA 
OF  THE  MOON'S 
SHADOW. 


Fig.  117.  —  CROSS-SECTION 
OF  A  SHADOW. 


126  DESCRIPTIVE    ASTRONOMY. 

moon  passes  into  the  umbra,  and  partial  if  only  a  portion  of  the 
moon  is  immersed  in  the  umbra. 

172.  Phenomena  of  a  Total  Lunar  Eclipse.  — When  the  moon  is  in 
the  penumbra  of  the  earth's  shadow,  enough  sunlight  still  strikes  it 
to  make  it  shine  brightly ;   no  one  would  surmise  from  its  appear- 
ance that  it  was  about  to  suffer  eclipse.     But  as  soon  as  it  reaches. 

the  umbra,  the  portion  of  its  limb  in  the  dark 
shadow  disappears  from  view,  the  moon  having 
the  appearance  exhibited  in  Fig.  118.  The 
dark  notch  grows  until  the  entire  moon  is 
immersed  in  the  umbra.  But,  strange  to  say^ 
the  whole  moon  usually  becomes  visible,  shin- 
ing with  a  dull  copper-colored  light.  The 
explanation  is  not  far  to  seek.  Many  of  the 
sun's  rays  pass  through  the  earth's  atmos- 
here  ^  c  and  D  (pj  „-)  and,  being  re- 

TOTAL  LUNAR  ECLIPSE. 

fracted,  pass  into  the   umbra   and  light  up  the 

moon  with  the  sunset  tinge.  Should  the  earth's  atmosphere  be 
charged  with  clouds  where  the  sun's  rays  attempt  to  struggle 
through  it,  the  sunlight  will  be  stopped  by  the  clouds,  and  the 
moon  will  be  entirely  invisible ;  this  happens  rarely.  When  the 
forward  edge  of  the  moon  emerges  from  the  umbra,  totality  is  past ; 
the  eclipse  ends  when  the  entire  moon  has  emerged  from  the  umbra. 
Any  phase  of  a  lunar  eclipse  is  visible  from  the  whole  of  that  hemi- 
sphere of  the  earth  which  is  turned  toward  the  moon. 

173.  Cause  of  a  Solar  Eclipse.  —  A    solar   eclipse    is    caused,    as 
shown  in  Fig.  116,  by  the  moon's  passing  between  the   earth  and 
the  sun,  so  as  to  obscure  the  sun  either  partially  or  wholly.     Since 
the  moon  does  not  move  in  the  plane  of  the  ecliptic,  it  does  not 
get  within  the  conical  space  ABCD   (Fig.  115),  every  month,  but 
usually  passes  above  or  below  it.     But  when  any  part  of  the  moon 
enters  this  conical  space,  the  sun  is  at  least  partially  obscured  at 
some  point  of  the  earth's  surface. 

174.  The  Moon's  Shadow.  —  In  Fig.  n 6,  the  moon's  shadow,  where 
it  falls  upon  the  earth,  is  quite  narrow.     On  account  of  the  variations 
of  the  earth's  distance  from  the  sun,  and  of  the  moon's  distance  from 
the  earth,  the  moon's  distance  from  the  sun  changes.     The  nearer  it 
is  to   the  sun,  the  shorter  is  its  shadow  (umbra)  :    the   farther  away,. 


THE    MOON    AND    ECLIPSES. 


— 


C/) 

m 


THE    MOON    AND    ECLIPSES. 


127 


the  longer  the  shadow.  Usually  the  shadow  is  not  quite  long  enough 
to  reach  the  earth's  surface.  Under  the  most  favorable  circum- 
stances, the  diameter  HK  (Fig.  116)  of  the  cross-section  of  the 
shadow  at  the  earth's  surface  is  168  miles.  The  penumbra  of  the 
moon's  shadow  is  shown  by  the  light  shading  in  Fig.  117.  The 
moon  hides  a  portion  of  the  sun  from  an  eye  situated  anywhere  in 
the  penumbra.  The  moon  moves  eastward,  but  as  the  earth  turns 
in  the  same  direction,  the  shadow  does  not  skim  over  the  c^  atinents 
as  fast  as  it  otherwise  would.  A  projectile  from  a  modern  rifled 
cannon  would  keep  up  with  it  for  a  few  seconds. 

175.  Varieties  of  Solar  Eclipses.  —  A  total  solar  eclipse  occurs 
when  the  whole  sun  is  hidden  from  view.  This  happens  only 
when  the  observer  is  within  the  umbra  of  the  moon's  shadow. 
Since  the  diameter  of  the  cross-section  of  the  umbra  at  the  earth 


Fig.  120.  —  PATH  OF  THE  CENTRAL  LINE  OF  THE  ECLIPSE  OF  MAY  27,  1900. 

is  always  less  than  170  miles,  the  path  of  the  shadow  on  the  earth's 
surface  is  long  and  narrow ;  a  total  eclipse  is  visible  only  to  those 
who  are  in  this  path.  Fig.  116  shows  that  the  penumbra  is  much 
wider;  the  average  diameter  of  its  cross-section  at  the  earth  is 
4,400  miles. 


128 


DESCRIPTIVE    ASTRONOMY. 


OF  THE   SUN  DURING 
AN  ANNULAR  ECLIPSE. 


For  any  one  situated  within  the  penumbra  there  will  be  a  partial 
eclipse.  The  nearer  he  is  to  the  true  shadow  path,  the  more  of  the 
sun  will  be  hidden.  The  next  total  solar  eclipse  visible  in  the  United 

States  occurs  on  May  27, 1900. 

The    path  of  the    shadow  is 

shown  in  Fig.  120.    When  the 

umbra  is  not  long  enough  to 

reach  the  earth,  any  one  at  R 

in  Fig.  121,  where  the  axis  of 

the  umbra  prolonged  cuts  the 

earth's  surface,  can  look  past  Fig.  122.  —APPEARANCE 

the  moon's   edge  and    see  a 

part  of  the  sun,  which  will  then 

have  the  appearance  shown  in  Fig.  122.     Such 

an  eclipse  is  called  annular. 

176.  Phenomena  of  Partial  and  Annular  Eclipses. 
—  At  the  beginning  of  the  eclipse,  the  moon 
appears  to  eat  away  the  edge  of  the  sun's  disk, 
forming  a  notch  similar  to  that  shown  in  Fig. 
118  for  a  lunar  eclipse.     The    notch   increases 
to  its  maximum  size,  and  then  diminishes.     One 
may  get   a   good    idea    of  the    appearance    by 
taking  two  equal  circles,  one  black,  the  other 
white,  and    passing  the  black  one  slowly  over 
the  face  of  the  white  one,  leaving  a  greater  or 
less  portion  of  the  white  one  exposed  to  view. 
To  represent  an  annular  eclipse,  the  black  circle 
must  be  smaller  than  the  white  one. 

With  a  telescope  the  lunar  mountains  are 
easily  seen,  projecting  from  the  moon's  limb 
where  it  eats  into  the  sun. 

177.  Phenomena   of   Total   Eclipses.  —  A   total 
eclipse  is  perhaps  the  grandest  of  natural  phe- 
nomena.    It  begins  in  the  same  way  as  a  partial  one ;   just  before 
the  sun  is  entirely  covered,  the  landscape  assumes  an  unearthly  hue. 
Awe  seizes  the  beholder:   one  sometimes  sees  the  moon's  shadow 
advancing  through  the  air  with  terrifying  swiftness,  as  if  to  smite 
him.     In  a  few  seconds  it  reaches  him,  and  the  last  ray  of  sunlight 


Fig.  121.  —  CAUSE  OF  AN 
ANNULAR  ECLIPSE. 


THE    MOON    AND    ECLIPSES.  I2Q 

is  gone;  the  planets  and  bright  stars  appear.  Around  the  black 
ball  now  hanging  in  the  sky  the  pearly  corona  flashes  out  in  all  its 
weird  beauty.  At  its  base  glow  the  prominences,  like  rubies  set  in 
pearl.  Men's  faces  grow  ghastly.  The  silence  of  death  is  upon  the 
beholders.  Soon  there  is  a  sudden  flash  of  sunlight  at  the  western 
limb  of  the  moon :  the  corona  and  prominences  fade  apace. 

The  gloom  is  overpast,  and  silence  gives  place  to  exclamations  of 
wonder  and  delight. 

178.  Observations  During  Totality.  —  Some  of  the  more  important 
of  the  observations  made  by  astronomers  are  given  below. 

1.  Photographs  of  the  corona  and  prominences  are  taken. 

2.  The  structure  of  the  inner  portions  of  the  corona,  which  can  be 
seen  only  during  a  total  eclipse,  is  carefully  studied  with  the  telescope. 

3.  Spectroscopic  observations  are  made  on  the  corona,  the  pro- 
tuberances, and  the  low-lying  regions  of  the  chromosphere. 

4.  Search  is  prosecuted  for  possible  small  planets  near  the  sun : 
it  is  claimed  that  such  objects  were  seen  during  the  eclipse  of  July 
29,  1878,  by  two  American  astronomers.1     Diligent  search  has  been 
made  for  these  during  more  recent  eclipses,  but  without  success. 

5.  Drawings  are  made  of  the  outer  corona  to  determine  its  extent 
and  boundaries. 

179.  Duration  and  Number  of  Eclipses.  —  An  eclipse  of  the  moon, 
if  total,  may  last  for  four  hours.     During  half  this  time,  the  whole  of 
its  disk  will  be  in  eclipse. 

A  total  solar  eclipse,  from  first  to  last  contact,  occupies  about 
two  hours.  Totality  may,  on  the  rarest  occasions,  last  nearly  eight 
minutes.  Its  duration  is  ordinarily  only  two  or  three  minutes.  In 
some  years  there  are  no  lunar  eclipses :  three  may  occur  in  a  year, 
as  will  happen  in  1898. 

Every  year  there  are  at  least  two  solar  eclipses  ;  there  may  be  five. 

The  greatest  number  of  eclipses  that  can  occur  in  any  year  is 
seven,  of  which  two  are  lunar. 

NOTE.  —  The  effects  of  a  total  solar  eclipse  on  animals  are  interesting.  Bees 
return  to  the  hive.  Chickens  go  to  roost.  Caged  birds  put  their  heads  under 
their  wings.  Bats  and  owls  fly  out  of  their  accustomed  retreats.  Dogs  are  terri- 
fied, and  sometimes  howl  dismally.  Horses  have  been  known  to  lie  down  in  the 
public  highway  and  refuse  to  advance.  Some  oxen  were  once  seen  to  range  them- 
selves in  a  circle,  back  to  back,  with  horns  outward,  as  if  to  resist  an  attack. 

1  Lewis  Swift  and  James  C.  Watson. 
9 


I3O  DESCRIPTIVE    ASTRONOMY. 


EXERCISES. 

180.    i.  If  the  moon  should  cease  to  rotate  on  its  axis,  would   its 
entire  surface  become  visible  to  us? 

2.  When  the  moon  is  new,  does  it  rise  and  set  at  about  the  same 
time  that  the  sun  does? 

3.  (a)  When  the  moon  is  full,  where  is  it  to  be   looked  for  just 
after  sunset? 

(b)  Where  just  before  sunrise? 

(c)  Where  at  noon? 

(d)  Where  at  midnight? 

4.  (a)  When  the  moon  becomes  a  crescent,  shortly  after   being 
new,  does  it  set  a  little  while  before  the  sun,  or  after  it? 

(b)  Does  it  rise  before  the  sun,  or  after? 

5.  When  the  moon  is  at  its  first  quarter,  does  the  terminator  (the 
straight  edge)  lie  on  the  left  hand  side  of  the  illuminated  portion,  or 
on  the  right  hand  side,  as  we  look  at  it? 

6.  When  the  moon  is  at  its  first  quarter,  does  it  cross  the  meridian 
a  few  hours  before  the  sun,  or  a  few  hours  after? 

7.  When  the  moon  is  at  its  first  quarter,  and  the  sun  is  setting, 
do  we  look  in  the  south  for  the  moon,  or  in  the  east? 

8.  Does  the  full  moon  shine  all  night,  if  the  sky  is  clear? 

9.  (a)  When  the  moon  is  in   its  last  quarter,  in  what  direction 
(north,  east,  south,  or  west)  is  it  to  be  seen  at  sunrise? 

(b)    In  what  direction  at  sunset? 

10.  On  the   ecliptic   are    four    cardinal    points;    viz.  the    vernal 
equinox,  the  summer  solstice,  the  autumnal  equinox,  and  the  winter 
solstice.     (§§  98,  99.) 

(a)  The  sun  being  in  the  vernal  equinox,  if  the  moon  were  full, 
near  what  point  of  the  ecliptic  would  it  be? 

(b)  Where,  if  at  first  quarter? 

(c)  Where,  if  at  last  quarter? 

11.  If  the  moon  on  a  given  night  be  near  either  equinox,  near 
what  points  of  the  horizon  will  it  rise  and  set? 

12.  The  moon  on  a  given  night  is  near  the  summer  solstice. 

(#)    Will  it,  as  seen  from  your  home,  rise  in  the  northeast,  or  in 
the  southeast? 


THE    MOON    AND    ECLIPSES. 

($)  When  crossing  the  meridian,  will  it  be  near  the  zenith,  or 
low  down  near  the  southern  horizon? 

13.  Is  the  moon  visible  an  hour  after  sunrise,  when  it  is  at  the 
last  quarter? 

14.  If  the  moon  be  full  about  Christmas  time,  will  it  run  high 
(that  is  cross  the  meridian  near  the  zenith),  or  low? 

[To  answer  this  question  first  find,  from  the  time  of  year,  where 
the  sun  is  in  the  ecliptic ;  then  from  the  relative  positions  of  the  sun 
and  moon  determine  what  point  of  the  ecliptic  the  moon  is  near.] 

15.  Why  is  not  the   dark   part   of  the   moon   rendered    plainly 
visible  by  earth  shine  at  the  first  quarter,  as  well  as  when  the  moon 
is  a  narrow  crescent? 

1 6.  A  mariner  computes  that  upon  a  certain  date  the  moon  will 
occult  the  bright  star  Aldebaran,  the   disappearance   occurring   at 
8  h.  26  m.  47  sec.,  Greenwich  mean  time.     By  observing  the  occul- 
tation,  he  finds  that  his  chronometer  reads  8  h.  25  m.  58  sec.  at  the 
time  of  the  star's  disappearance.     Is  his  chronometer  fast  or  slow, 
and  how  mnch? 

17.  Can  a  lunar  crater  ever  be  filled  with  the  shadow  of  its  own 
wall,  so  that  the  bottom  of  the  crater  will  be  invisible  to  us? 

1 8.  Why  are  not  the  rays  radiating  from  lunar  craters  overflows 
of  lava? 

19.  How  does  the  atmosphere  prevent  our  seeing  stars  by  day 
as  well  as  we  see  them  at  night? 

20.  The  lines  AD  and  BV  in  Fig.  115  are  tangent  to  the  circle  E. 
Are  they  tangent  at  exactly  the  same  point? 

21.  If  the  moon  were  as  large  as  the  earth,  would  it  ever  suffer 
a  total  eclipse  ? 

22.  (a)  At  what  kind   (§175)  of  a  solar  eclipse  does  the  moon 
look  smaller  than  the  sun? 

(b)    At  what  kind  does  it  look  larger? 

23.  If  the  earth  were  suddenly  robbed  of  its  atmosphere  when 
the  moon  was  visible  during  a  total  lunar  eclipse,  what  change  would 
there  be  in  the  moon's  appearance? 

24.  In  a  daily  paper,  of  wide  circulation,  the  head  lines  of  an 
article  on  a  solar  eclipse  read  thus :  "  The  Moon  Casts  its  Shadow 
on  the  Sun."  Change  that  sentence  so  that  it  will  express  the 
truth. 


132  DESCRIPTIVE    ASTRONOMY. 

25.  When  an  observer  at  Chicago  sees  an  annular  eclipse,  can 
an  observer  in  some  other  city  see  the  eclipse  as  total? 

26.  (V)  When  a  solar  eclipse  is  partial  for  one  observer,  may  it 
be  total  for  another  at  the  same  time  ? 

(b)   When  a  lunar  eclipse  is  partial  for  one  observer,  may  it  be 
total  as  seen  by  another  at  the  same  time  ? 

27.  The  total  solar  eclipse  of  January,  1889,  was  visible  in  both 
California  and  Nevada.     Did  an  observer  on  the  coast  of  California 
see  it  before  or  after  one  in  Nevada  saw  it? 

28.  Does  a  total   lunar  eclipse  end  for    an  observer  in   Boston 
at  the  same  instant  as  for  one  in  Chicago  ? 

29.  What  is  the  derivation  of  the  word  "  annular"? 

30.  (#)  Do  lunar  eclipses  happen  when  the  moon  is  new? 
(£)    Do  solar  eclipses  happen  when  the  moon  is  new? 


MOTIONS    OF   THE    PLANETS.  133 


CHAPTER   VIII. 

MOTIONS   OF   THE   PLANETS. 

"  'T  is  by  the  secret,  strong  attracting  force,  . 

As  with  a  chain  indissoluble  bound, 
The  system  rolls  entire." 

THOMSON. 

181.  Their  Orbits.  — The  orbit  of  each  planet  is  an  ellipse  (§96), 
one  focus  of  which  is  in  the  sun.     The  planes  of  all  the  planetary 
orbits,  excepting  those  of  some  of  the  asteroids  (§  224),  are  but  little 
inclined  to  the  ecliptic.     If  a  dot  were  placed  in  the  centre  of  this 
page,  to  represent  the  sun,  and  all  the  planetary  orbits  were  accu- 
rately drawn  around  it,  the  deviation  of  any  one  of  them  from  true 
circularity  would  not  be  perceived.     The  definitions  of  major  axis, 
minor  axis,  perihelion,  aphelion,  and  mean  distance,  given  in  §  96, 
apply  to  the  orbit  of  any  planet.     The  radius  vector  of  a  planet  is  a 
line  drawn  from  the  focus  of  its  orbit  to  the  planet's  centre. 

182.  Motion   in   Orbit.  —  If  one    could    station    himself  in    space 
between  the  north  star  and  the  sun,  and  a  billion  miles  from  the 
latter,  on  looking  back  at  the  planets  he  would  see  that  they  were 
all  moving  about  the  sun  in  a  direction  opposite  to  that  of  the  hands 
of  a  watch.     This  is  an  easterly  motion.     When  a  planet  is  at  peri- 
helion, it  moves  more  swiftly  than  at  any  other  point  of  its  orbit. 
The  planet  nearest  the  sun  moves  more  rapidly  than  any  other. 

183.  Newton's  Law  of  Gravitation.  —  This  law,  to  which  all  bodies 
in  the  universe  are  supposed  to  be  subject,  may  be   stated   in   the 
following  way.      The  mutual  attraction  between  any  two  particles  is 
proportional  to  the  product  of  their  masses,  and  inversely  proportional 
to  tiie  square  of  the  distance  between  tJiem. 

This  law  may  be  expressed  as  an  algebraic  equation :  let  m  and 
m'  be  the  masses  of  two  bodies,  d  the  distance  between  them,  and  k 
a  number,  the  value  of  which  depends  on  the  units  of  mass  and  dis- 
tance employed. 

.  mm1 
Attraction  =  k . 


134  DESCRIPTIVE    ASTRONOMY. 

To  make  this  clearer,  suppose  that  two  lead  balls  a  mile  apart 
attracted  each  other  with  a  force  of  an  ounce.  If  one  ball  were 
suddenly  made  five  times  as  massive,  the  resulting  attraction  would 
be  five  times  as  great  as  before.  If  at  the  same  time  the  other  were 
made  three  times  as  massive,  the  new  attraction  between  the  bodies 
would  not  be  5  +  3,  or  8  times  the  old  attraction,  but  5  X  3,  or  15 
times  the  old  attraction.  Again,  suppose  that  the  masses  of  the 


Fig.  123.  —  SIR  ISAAC  NEWTON. 

balls  remained  the  same  as  at  first,  but  the  distance  between  the 
balls  was  doubled,  the  new  attraction  would  not  be  |  of  the  old,  but 
the  square  of  |,  which  is  J,  of  the  old. 

The  force  of  gravity  binds  each  planet  to  the  sun. 

184.  What  keeps  the  Planets  Moving?  —  Persons  ignorant  of  the 
laws  of  mechanics  frequently  think  that  gravity  alone  cannot  keep 
the  planets  moving,  but  that  some  other  force  is  pushing  them. 
But  there  is  no  such  extra  pushing  force.  One  of  the  laws  of 
mechanics  is  that  a  body  once  set  in  motion  will  continue  to  move 


MOTIONS    OF    THE    PLANETS. 


135 


in  a  straight  line  with  a  uniform  velocity,  unless  ac^^^K  by  some 
external  force.  The  tendency  to  keep  going  if  once  .set  in  motion, 
or  to  remain  at  rest  if  stopped,  is  known  as  inertia.  So  a  planet 
needs  no  pushing  force  behind  it :  having  in  some  way  been  set  in 
motion,  its  tendency  is  to  move  in  a  straight  line ;  but  the  gravita- 
tional pull  of  the  sun  compels  it  to  describe  a  curve  instead. 


.;: 


Fig.  124.  —  KEPLER. 

185.  Kepler's  Laws.  —  Before  the  time  of  Kepler,  who  was  born 
In  1571,  the  heavenly  bodies  were  supposed  to  move  in  circles.  He 
discovered  three  laws  concerning  the  motions  of  the  planets  :  — • 

I.  The  orbit  of  each  planet  is  an  ellipse,  the  sun  being  at  one  of 
its  foci. 

II.  The  radius  vector  of  a  planet  describes  equal  areas  in  equal 
times. 


136 


DESCRIPTIVE    ASTRONOMY. 


III.  The  squares  of  the  times  of  revolution  of  any  two  planets 
are  to  each  other  as  the  cubes  of  their  mean  distances  from  the  sun. 

The  second  law  is  illustrated  in  Fig. 
125.  If  a  planet  describes  the  arcs  AB 
and  CD  in  the  same  length  of  time,  the 
area  of  SAB  is  equal  to  that  of  SDC. 

Let  t  and  f  denote  the  times  of  revo- 
lution of  two  planets,  while  a  and  a1  are 
their  mean  distances.  Then,  by  the  third 
law,  ^://2::fl8:a/8. 

The  discovery  of  this  law,  known  as 
the  harmonic  law,  after  seventeen  years  of 
arduous  labor,  caused  Kepler  the  great- 
est exultation.  He  wrote  concerning  it: 
"  The  die  is  cast :  the  book  is  written, 
to  be  read  either  now  or  by  posterity,  — 
I  care  not  which.  It  may  well  wait  a 
century  for  a  reader,  as  God  has  waited  six  thousand  years  for  an 
observer." 

186.  Perturbations.  —  Gravitation  being  universal,  it  follows  that 
the  planets  attract  each  other.  These  attractions  cause  disturbances 
of  their  elliptic  motions.  The  computation  of  these  perturbations 
has  taxed  the  highest  skill  of  mathematical  astronomers ;  by  a  series 
of  profound  and  elegant  researches  it  has  been  proved  that  the 
stability  of  the  planetary  system  is  not  endangered  by  them. 


Fig.  125.  —  EQUAL  AREAS  IN 
EQUAL  TIMES. 


APPARENT    MOTIONS. 

187.  Two  Classes  of  Planets. —  For  convenience  in  discussing  their 
apparent  motions  the   planets   are   divided  into   two   classes.     The 
inferior  planets  are  those  the  orbits  of  which  lie  within  that  of  the 
earth :    these   are   Mercury  and  Venus.     The   superior  planets    are 
those  whose  orbits  are  exterior  to  that  of  the  earth :   they  are  Mars, 
the  asteroids,  Jupiter,  Saturn,  Uranus,  and  Neptune. 

188.  Aspects.  —  One  is  aided  in  remembering  the  following  ex- 
planations of  the  aspects  of  the  planets  by  the  thought  that  they  all 
refer  to  the  position  of  a  planet  with   relation  to   the  sun,  as  we, 
looking  out  from  the  earth,  see  the  two  bodies. 


MOTIONS    OF    THE    PLANETS. 


137 


Conjunction  1  occurs  when  a  planet  appears  to  be  close  to  the 
sun.  A  superior  planet  is  then  beyond  the  sun.  An  inferior  planet 
may  be  beyond  the  sun,  in  which  case  it  is  in  superior  conjunction, 
or  it  may  be  between  the  earth  and  the  sun ;  in  that  case  it  is  in 
inferior  conjunction.  Conjunction  is  indicated  by  the  sign  &  . 

Opposition  occurs  when  the  planet  is  in  the  opposite  direction 
(from  us)  to  that  in  which  the  sun  lies.  If  the  sun  were  neaa*  the 


OPPOSITION 

Fig.  126.  —  ASPECTS  OF  THE  PLANETS. 

east  point  of  the  horizon,  a  planet  in  opposition  would  be  near  the 
west  point.     Opposition  is  denoted  by  the  sign  8 . 

A  planet's  elongation  from  the  sun  is  the  angle  formed  at  the 
earth  by  two  lines  drawn  from  it  to  the  planet  and  the  sun  respect- 

1  There  are  different  kinds  of  conjunction,  opposition,  etc.,  because  the  planets' 
orbits  do  not  coincide  with  the  ecliptic.  Planets  are  at  conjunction  in  longitude  when 
their  longitudes  are  the  same:  they  are  in  conjunction  in  right  ascension  when  their 
right  ascensions  are  the  same. 


138  DESCRIPTIVE    ASTRONOMY. 

ively.     The   greatest  elongation  of  an  inferior  planet  is  illustrated  in 
Fig.  126. 

A  superior  planet  is  in  quadrature  when  its  elongation  is  90°. 
Quadrature  is  denoted  by  the  sign  n . 

189.  Apparent  Movement  of  an  Inferior  Planet.  —  In  Fig.  1 26  the 
orbit  of  Venus  is  drawn  to  illustrate  that  of  an  inferior  planet.     After 
being  at  inferior  conjunction,  Venus,  moving  in  a  direction  opposite 
to  the  hands  of  a  watch,  goes   to  its  greatest  western  elongation. 
If  the  earth  stood  still,  Venus  would  arrive  at  its  elongation  in  four 
weeks  ;   but  as  the  earth  chases  after  it,  the  interval  between  inferior 
conjunction  and    greatest  western   elongation  is   lengthened   to   2^ 
months.     The  planet  then  passes  on  through  superior  conjunction 
and  greatest  eastern  elongation  to  inferior  conjunction  again. 

While  it  is  travelling  from  inferior  to  superior  conjunction,  a  man 
facing  the  sun  will  see  the  planet  at  the  right  (or  west)  of  the  sun. 
During  the  other  half  of  its  course  it  will  be  east  of  the  sun. 

190.  Apparent  Movement  of  a  Superior  Planet.  —  In  Fig.   127   are 
represented  the  orbits  of  the  earth  and  of  Mars,  P,  P',  and  P"  being 
on  the  celestial  sphere.     E  and  M  are  the  positions  of  the  earth  and 
Mars  when  the  latter  is  at  opposition.     Mars  then  appears  to  be  at 
the  point  P    on   the    celestial    sphere.     Two    weeks    thereafter  the 
earth  has  moved  to  E',  and  Mars,  moving  more  slowly  on  account  of 
its  greater  distance  from  the  sun,  has  traversed  the  arc  M  M'.     Mars 
then  appears  to  be  at  P',  which  is  west  of  P.     So  a  superior  planet, 
though  really  moving  eastward  in  its  orbit,  appears  when  near  oppo- 
sition to  move  toward  the  west  among  the  stars,  because  its  motion 
is  slower  than  that  of  the  earth,  and  the  two  bodies  are  moving  in 
nearly  the  same  direction.     This  westward  motion  is  said  to  be  the 
retrograde. 

Again',  let  Mars  at  M  be  in  conjunction,  the  earth  being  at  E", 
and  Mars  appearing  to  us  to  be  at  P.  In  a  few  days  the  earth  ar- 
rives at  E"'  and  Mars  at  M',  so  that  it  appears  to  be  at  P",  having 
moved  east  from  P.  Had  the  earth  remained  at  E",  Mars  in  moving 
from  M  to  M'  would  have  appeared  to  go  east,  but  would  not  have 
reached  P". 

Summing  the  matter  up,  we  reach  three  conclusions :  — 
I.  If  the  earth  were  stationary,  the  planet's  easterly  motion  in  its 
orbit  would  cause  it  to  appear  to  move  eastward  among  the  stars. 


MOTIONS    OF   THE    PLANETS. 


139 


II.  When  the  planet  is  near  opposition,  the  more  rapid  motion 
of  the  earth  causes  its  eastward   motion  to  be  apparently  reversed, 
so  that  it  retrogrades  or  moves  westward  among  the  stars. 

III.  When  the  planet  is  near  conjunction,  its  apparent  eastward 
motion  among  the  stars  is  swifter  than  it  would  be,  were  the  earth  at 
rest. 


Fig.  127.  —APPARENT  MOVEMENT  OF  A  SUPERIOR  PLANET. 

When  changing  its  apparent  eastward  motion  to  a  westward,  and 
vice  versa,  the  planet  is  said  to  be  at  a  stationary  point. 

191.  Evening  and  Morning  Stars. — When  a  planet  rises  between 
midnight  and  the  ensuing  sunrise,  it  is  called  a  morning  star.  When 


140  DESCRIPTIVE    ASTRONOMY. 

it  is  above  the  horizon  at  some  instant  between  sunset  and  the  fol- 
lowing midnight,  it  is  an  evening  star.  Hence  an  evening  star  can 
be  seen  before  midnight  (if  not  too  near  the  sun),  and  a  morning 
star  cannot. 

192.  Periods,  Sidereal  and  Synodic.  —  The    sidereal    period    of  a 
planet   is   the   time  of  a  complete  revolution  around   the   sun.     The 
synodic  period  is  the  time  which  elapses  between  two  successive 
conjunctions  of  the  planet  with  the  sun.     If  the  planet  be  an  inferior 
one,  the  two  conjunctions  must  be  both  inferior  or  both  superior. 

EXERCISES. 

193.  i.   (a)  At  what  point  of  its  orbit  is  a  planet,  when  its  radius 
vector  has  its  greatest  length  ? 

(b)  At  what  point  when  it  is  the  shortest? 

2.  (a)  Why  does  a  planet  move  most  swiftly  when  at  its  peri- 
helion? 

($)   Why  does  Mercury  travel  more  miles  in  an  hour  than  any 
other  planet? 

3.  Two  balls,  a  rod  apart,  attract  each  other  with  a  force  of  one 
grain.     If  the  mass  of  one  ball  be  made  ten  times  as  great,  while  that 
of  the  other  is  halved,  what  will  be  the  attraction  between  them,  the 
distance  remaining  the  same? 

4.  In  exercise  3  what  would  have  been  the  mutual  attraction  had 
the  original  balls  been  placed  ten  rods  apart? 

5.  What  would  have  been  the  mutual  attraction  had  the  original 
balls  been  placed  one  fourth  of  a  rod  apart? 

6.  What  would  have  been  the  attraction,  if  each  ball  had  been 
halved,  and  the  distance  had  been  halved  also? 

7.  What  would  have  been  the  attraction  if  the  mass  of  one  ball 
had  been  made  three  times  as  great,  while  that   of  the  other  was 
made  ten  times  as  great,  and  the  distance  between  them  shortened 
to  one  fifth  of  a  rod  ? 

8.  If  the  gravitational  pull  between  the  earth  and  the  sun  were 
suddenly  to  cease,  how  would  the  former  move? 

9.  How  does  Kepler's  second    law  show  that  a  planet  when  at 
aphelion  must  describe  a  shorter  arc  of  its  orbit  in  a  day  than  when 
at  perihelion? 


MOTIONS    OF    THE    PLANETS.  141 

10.  The  mean  distance  of  the  earth  from  the  sun  being  93  millions 
of  miles,  while  that  of  Jupiter  is  483  millions,  show  by  Kepler's  third 
law  that  the  period  of  Jupiter's  revolution  about  the  sun  is  nearly 
twelve  years. 

11.  The  mean  distance  of  Neptune  being  thirty  times  that  of  the 
earth,  show  that  its  period  is  over  164  years. 

12.  The  period  of  Uranus  being  eighty-four  years,  show  that  its 
mean  distance  from  the  sun  is  over  nineteen  times  that  of  the  earth. 

13.  Does  a  superior    planet  which    is  in  conjunction  set    about 
sunrise? 

14.  Does  a  planet  when  in  opposition  rise  about  sunset? 

15.  (a)  At  about  what  time  of  day  does  an  inferior  planet,  when 
in' inferior  conjunction,  cross  the  meridian? 

(b}  At  what  time,  when  in  superior  conjunction? 

1 6.  What    is  the  aspect  of  a  superior    planet   which    is  on    the 
meridian  at  midnight? 

17.  Why  does  a  superior  planet  look  brightest  when  at   oppo- 
sition ? 

1 8.  Why  are  planets  not  easily  observed  when  they  are  in  con- 
junction ? 

19.  When  Mercury  is  at    its  greatest    eastern  elongation,  being 
28°  from  the  sun,  is  it  visible  in  the  evening  twilight? 

20.  If  Mercury  was  seen  going  across  the  face  of  the  sun,  would 
it  move  from  the  sun's  eastern    limb  towards    its  western,  or   vice 
versa  ? 

21.  If  Venus  is  at  its  greatest  eastern  elongation,  being  47°  from 
the  sun,  does  it  rise  before  the  sun? 

22.  When  at  its  greatest  western  elongation,  is  Venus  a  morning 
star  or  an  evening  star,  or  both? 

23.  Just  after  an  inferior  planet  passes  its  superior  conjunction,  is 
it  a  morning  star  or  is  it  an  evening  star? 

24.  If  Venus  is  at  its  eastern  elongation,  does  it  cross  the  upper 
branch  of  the  meridian  in  the  forenoon,  or  in  the  afternoon? 

25.  Could  Jupiter  and  Venus  ever  appear  to  be  close  together? 

26.  At  one  of  its  oppositions.  Mars  was  near   perihelion,  while 
the  earth    was    near    aphelion.      At    another  opposition  Mars  was 
near  aphelion,  while  the  earth  was  near   perihelion.     At  which  of 
the  two  oppositions  could  Mars  be  best  seen  by  us? 


142  DESCRIPTIVE    ASTRONOMY. 

27.  When    does  a  superior    planet  appear    to  have  the  smaller 
diameter,  at  opposition  or  at  conjunction? 

28.  When  does  an  inferior  planet    appear    to    have  the  smaller 
diameter,  at  inferior  conjunction  or  at  elongation? 

29.  Should  an    inferior    planet,  shining    by  reflecting    the  sun's 
light,  show  phases  similar  to  those  of  the  moon? 

30.  Draw  a  picture  containing  two  concentric  circles,  one  repre- 
senting the  orbit  of  the  earth,  the  other  that  of  a  superior  planet. 
Mark  the  positions  of  the  sun,  earth,  and  planet,  when  the   planet 
is  in  conjunction  with  the  sun.       Determine  whether  the  synodic 
period  of  the  planet  is  longer  than  the  sidereal. 

31.  Find,  by  making  a  drawing  similar  to  that  described  in  the 
preceding  exercise,  whether  the  sidereal  period  of  an  inferior  planet 
is  longer  than  the  synodic. 


MERCURY,    VENUS,    MARS,    THE    ASTEROIDS. 


143 


CHAPTER    IX. 

MERCURY,    VENUS,    MARS,    THE   ASTEROIDS. 

"  Now  glowed  the  firmament 
With  living  sapphires :  Hesperus,  that  led 
The  starry  host,  rode  brightest." 

MILTON. 

194.  Two  Groups  of  Planets.  —  When  the  planets  themselves  are 
considered,  instead  of  their  orbits,  they  fall  naturally  into  two  divis- 
ions. Mercury,  Venus,  the  earth,  and  Mars  are  all  comparatively 
small  bodies,  are  doubtless  solid,  and  are  quite  dense.  Each  is  sup- 


Fig.  128.  —  RELATIVE  SIZES  OF  THE  PLANETS. 

posed  to  have  an  atmosphere  of  small  mass  compared  with  the  mass 
of  the  body  enveloped  by  it.  Their  equipment  of  moons  is  meagre. 
The  relative  sizes  of  the  planets  are  shown  in  Figure  128. 

Jupiter,  Saturn,  Uranus,  and  Neptune,   in  comparison  with  the 
other  planets,  are  giants  in  size.     But  their  densities,  are  small,  so 


144  DESCRIPTIVE    ASTRONOMY. 

that  they  have  perhaps  only  little  kernels  of  solid  matter  at  their 
centres.  Their  atmospheres  are  very  extensive  and  dense.  They 
are  liberally  provided  with  satellites,  except  Neptune.  Were  he 
nearer,  we  might  discover  that  he  had  a  goodly  retinue  of  them. 
The  planets  just  mentioned,  eight  in  all,  are  called  the  major  planets. 
The  minor  planets,  or  asteroids,  are  quite  insignificant  in  point  of 
size.  Very  little  is  known  of  their  physical  constitution. 


MERCURY,    £. 

195.  Distance  and  Diameter.1  —  The    mean    distance    of    Mercury 
from    the    sun    is    36,ooopoo    miles.     The    actual    distance    varies 
7,500000  miles  each  side   of  this  value,  the  orbit  being  much  more 
eccentric  than  that  of  any  other  of  the  large  planets. 

The  diameter  of  Mercury  is  3,000  miles. 

196.  Revolution  and  Rotation. — The  sidereal  period  is  88  days, 
so  that  it  performs  a  revolution  in  less  than  one  fourth  of  the  time 
required  by  the  earth. 

The  time  of  its  rotation  upon  its  axis  cannot  be  said  to  be  cer- 
tainly known.  One  astronomer,  a  century  ago,  thought  he  saw 
certain  appearances  on  the  planet  due  to  the  presence  of  high 
mountains ;  by  observing  them,  he  obtained  a  rotation  period  of 
about  24  hours.  But  his  observations  have  not  been  confirmed  by 
more  powerful  telescopes. 

Schiaparelli,  a  distinguished  Italian  astronomer,  who  has  made  a 
special  study  of  some  very  faint  markings  on  Mercury  since  1881, 
has  concluded  that  Mercury  rotates  on  its  axis  in  the  same  time 
that  it  revolves  about  the  sun.  Thus,  as  our  moon  continually  pre- 
sents the  same  face  to  the  earth,  Mercury  turns  the  same  side  to  the 
sun.  Schiaparelli  announced  his  discovery  to  a  friend  in  1882  in  the 
following  lines :  — 

"  Cynthiae  ad  exemplum  versus  Cyllenius  axe 
Aeternum  noctem  sustinet,  atque  diem  : 
Altera  perpetuo  facies  comburitur  aestu, 
Abdita  pars  tenebris  altera  Sole  caret." 

1  The  distance,  diameter,  sidereal  period,  and  rotation  time  of  each  planet,  should  be 
thoroughly  committed  to  memory. 


MERCURY,    VENUS.  145 

197.  Transits.  —  When   Mercury  is    near   inferior  conjunction,  it 
sometimes  gets  into  line  between  the  earth  and  the  sun,  so  that  it  is 
seen  by  us  as  a  small  black  circle  crossing  the  solar  disk.     Thirteen 
transits  occurred  during  the  nineteenth  century,  the  last  one  being 
on  Nov.  10,  1894. 

198.  Appearance  to  the  Naked  Eye. — The  planet  keeps  so  close 
to  the  sun,  that  it  is  not  readily  seen  with  the  naked  eye.     The  times 
of  its  elongation  are  the  best  times  to  look  for  it:   it  can  be  well 
seen  about  a  week  before  elongation,  as  well  as  a  week  after.     The 
dates  of  elongation  are  given  in  the  Nautical  Almanac.     The  best 
conditions  for  seeing  it  in  the  evening  occur  at  those  eastern  elonga- 
tions which  happen  in  March  or  April.     It  then  appears  like  a  star 
of  exceptional  brilliancy,  near  the  western  horizon,  distinguishable 
in    strong   twilight,    and    conspicuous    as    soon    as    night    sets    in. 
Copernicus  is  said  never  to   have .  seen   it. 

199.  Telescopic  Appearance.  —  The  telescope  shows  that  the  planet 
has  phases  like  the  moon :   it  therefore  shines  by  reflecting  the  light 
of  the    sun.     When  near  inferior  conjunction  it  is  a  narrow  cres- 
cent, as  is  the  moon  when  new.     Its  phase  at  superior  conjunction 
is  like  that  of  the  full  moon.     Favorable  views  of  the  planet  are 
rare,  since   it  must  be  observed   either  during  the  daytime,   or  at 
night  when  it  is  near  the  horizon.     Faint  dark  markings  are  some- 
times seen  on  its  disk,  but  they  are  so  indistinct  that  their  nature 
can  only  be  guessed  at.     They  may  be  dark  plains,  like  those  on  the 
moon,  or  possible  lakes  or  seas. 

200.  Physical  Condition.  —  There  is  spectroscopic  evidence  of  the 
presence  of  water  vapor ;   from  this  we  conclude^  that  both  air  and 
water  are  to  be  found  on  the  planet.     But  it  is  probable  that  the 
atmosphere  is  not  as  dense  as  ours.     The   sun  shines  seven   times 
as  hotly  as  on  the  earth. 

VENUS,    9. 

201.  Morning  and  Evening  Star. — Venus  is  the  most  brilliant  of 
the   planets,   and  when  at  its   maximum    brightness    casts    distinct 
shadows  of  objects  at  night,  in  the  absence  of  the  moon  or  bright 
artificial  lights  near  at  hand.     It  is  then  visible  to  the  naked  eye  in 
full  daylight.     Because  of  its  brightness  it  has  received  the  distinct- 
ive appellations  of  the  Evening  Star  and  the  Morning  Star.      The 


146  DESCRIPTIVE    ASTRONOMY. 

Greeks  called  it  Hesperus  when  it  was  an  evening  star,  and  Phos- 
phorus when  it  was  a  morning  star.  Of  late  years  many  ignorant 
people,  seeing  it  by  day,  have  supposed  it  to  be  a  reappearance  of 
the  Star  of  Bethlehem. 

202.  Distance  and  Diameter.  —  The  mean  distance  of  Venus  from 
the  sun  is  67,000000  miles ;  its  distance,  when  in  different  parts  of 
its  orbit,  varies  little,  because  its  orbit  is  more  nearly  circular  than 
that  of  any  other  planet.     The  planet's  diameter  is  7,700  miles:   it 
is  therefore  nearly  as  large  as  the  earth.     When  at  inferior  conjunc- 
tion it  is  nearer  to  us  than  any  other  planet  ever  is. 

203.  Revolution  and  Rotation.  —  Venus  accomplishes  a  revolution 
about  the  sun  in  225  days.     The  time  of  its  rotation  is  not  now  in 
much  doubt.     Until  lately  the  only  evidence  (and  that  very  insuffi- 
cient) was  that  it  revolved  in  23  h.  21  m.     But  the  recent  researches 
of  Schiaparelli,  the  keenness  of  whose  vision  is  remarkable,  render 
it  probable  that  the  time  of  rotation  is  225   days,  agreeing  with  the 
sidereal  period.     This  result  has   been  corroborated   by  two  other 
Italian  astronomers. 

204.  Transits.  —  Transits  of  Venus  across  the  sun's  face  are  much 
rarer  than  those  of  Mercury.     The  last  one  occurred  on  Dec.  6th, 
1882,  and  the  next  one  will  not  come  until  2004.     Another  is  due 
in  2012.     These  transits  have  a  high  degree  of  interest,  because  they 
have  been  used  in  finding  the  distance  of  the  earth  from  the  sun. 
Expensive  scientific  expeditions  have  been  sent  to  various  parts  of 
the  world,  by  the  governments  of  the  most  progressive  nations,  to 
observe  these  transits. 

Halley's  '  method  of  observation  consists  of  observing  the  times 
of  external  and  internal  contact  as  seen  from  two  stations  widely 
different  in  latitude.  As  shown  in  Fig.  129,  the  planet  as  seen  from 
these  stations  has  different  paths  across  the  sun's  disk.  The  dis- 
tance and  direction  of  one  station  from  the  other  being  known,  and 
the  lengths  of  the  two  paths  being  measured,  it  is  possible  by  trig- 
onometric methods,  too  difficult  to  be  explained  here,  to  find  the 
sun's  distance.  The  accuracy  of  the  final  result  depends  upon  the 
precision  with  which  the  times  of  contact  are  noted. 

Unfortunately,  when  the  black  circle  of  the  planet's  disk  is  inter- 
nally tangent  to  the  sun's  limb  at  the  beginning  of  the  transit,  it  has 

1  Halley  was  an  English  Astronomer  Royal  in  Newton's  time. 


VENUS. 


the  appearance  shown  in  Fig.  130.  As  it  moves  away  from  the 
limb,  it  is,  for  a  number  of  seconds,  apparently  attached  to  it  by  a 
black  ligament,  called  the  "  black  drop."  The  ligament  stretches, 
contracts,  and  finally  breaks.  Thus  it  is  very  difficult  to  note  the 


VENU5 


BLACK 


DROP 


Fig.  129.  —  A  TRANSIT  OF  VENUS. 


Fig.  130. — THE  BLACK  DROP. 


time  of  real  internal  contact.  The  action  of  the  planet's  atmos- 
phere also  vitiates  the  accuracy  of  the  observation.  A  phenomenon 
similar  to  the  black  drop  may  be  seen  by  placing  the  thumb  and 
forefinger  close  together,  and  holding  them  six  inches  or  less  from 
the  eye. 

On  account  of  these  troubles  astronomers  now  place  more  re- 
liance upon  other  methods. 

205.  Phases  and  Maximum  Brightness.  —  The  phases  of  Venus 
are  similar  to  those  of  Mercury  and  the  moon.  They  are  almost 
visible  to  the  naked  eye.  A  good  spyglass  brings  out  the  crescent 
phase  well.  The  time  of  greatest  brightness  does  not  occur  when 
the  planet  looks  like  a  full  moon,  for  then  it  is  farthest  from  us.  It 
comes  during  the  crescent  phase,  five  weeks  before,  and  the  same 
time  after,  inferior  conjunction. 

The  discovery  of  the  phases  of  Venus  was  one  of  the  first  fruits 
of  the  invention  of  the  telescope.  Galileo  l  made  the  discovery  and 
announced  it  in  the  following  anagram:  "  Haec  immatura  a  me  jam 
frustra  leguntur,  o.  y."  This  he  afterwards  transposed  so  that  it 
read,  "  Cynthiae  figuras  aemulat  mater  amorum." 

1  Galileo  Galilei  (1564-1642),  the  famous  Italian  philosopher. 


148 


DESCRIPTIVE    ASTRONOMY. 


206.    Telescopic  Appearance.  —  The  planet  is  of  dazzling  splendor, 
even  in  a  small  telescope ;   it  looks  almost  as  if  made  of  quicksilver, 


Fig.  131.  —  GALILEO. 

and  is  surrounded  by  a  marked  purplish  aureole  caused  by  the 
lack  of  achromatism  (§  39)  of  the  telescope. 

On  rare  occasions  ill  defined  spots  of  a  leaden  hue  are  seen  on 
its  surface.  They  may  be  continents  or  seas  dimly  descried.  Cer- 
tain very  bright  spots,  said  by  some  to  be  occasionally  visible  near 
the  limb,  have  been  thought  to  be  due  possibly  to  polar  ice  and 
snow. 

207.  Atmosphere.  —  When  Venus  is  near  inferior  conjunction, 
being  a  very  slender  crescent,  the  horns  or  cusps  of  the  crescent 
appear  to  be  much  prolonged,  so  that  they  really  surround  the 
dark  disk  of  the  planet.  When  about  to  enter  upon  a  transit,  a  ring 
of  light  is  seen  surrounding  the  entire  disk  (Fig.  132).  This  is 


VENUS,    MARS.  149 

the    sunlight   shining  through  the  planet's  atmosphere,  and    being 
refracted  by  it  to  our  eyes.     The  atmosphere  has  been  shown  to  be 


Fig.  132.  —  THE  RING  OF  LIGHT. 

denser  than  ours,  and  probably  less  than  twice  as  dense.    The  vapor 
of  water  has  been  detected  in  it  by  spectroscopic  observations. 

208.  Physical  Condition.  —  The    density    of    Venus   being   nearly 
equal  to  that  of  the  earth,  we  conclude  that  it  is  a  solid  body.     It 
probably  owes  its  brilliancy  to  the  fact  that  its  sky  is  almost  totally 
cloudy   at  all  times.     Any  one    looking  at  bright  white  masses  of 
cumulus  cloud  in  a  summer  sky  will  be  convinced  that  such  clouds 
reflect  light  much  better  than   the   general   landscape   does.     The 
excessive  cloudiness  in  turn,  combined  with  the  spectroscopic  evi- 
dence   of   water   vapor,   indicates  that  water   is    abundant   on    the 
planet's   face.      There  may  not  be   a   square  foot   of  dry  land  to 
vary  the  monotony  of  a  universal  ocean. 

MARS,    $. 

209.  Distance  and  Diameter. — The  mean  distance  of  Mars  from 
the  sun  is  141,500000  miles.     Its  orbit  is,  excepting  Mercury's,  the 
most  eccentric   of  all  the  orbits  of  the  major  planets,  so  that   the 
planet's  distance  varies   13,000000  miles   each    side  of  the   average 
distance.     Its  diameter  is  4,200  miles,  which  is  not  much  more  than 
half  the  earth's  diameter. 

210.  Revolution  and  Rotation.  —  The  sidereal  period  is  687  days, 
only  43  days  short  of  two  years.     By  comparison  of  drawings  of  the 


150  DESCRIPTIVE    ASTRONOMY. 

planet  made  soon  after  the  invention  of  the  telescope  with  those 
made  during  this  century,  the  rotation  time  has  been  determined 
with  great  precision.  It  is  24  h.  37  m.  22. 67  sec. 

211.  Appearance  to  the  Naked  Eye.  —  Mars,    being    a   superior 
planet,  is  best  seen  at  the  time  of  opposition,  when  it  is  near  the 
earth.     At  some  oppositions  it  comes  within  36,000000  miles  of  us, 
at  others  it  is  as  much  as  61,000000  miles  away.     This  variation  of 
distance  is  due  to  the  eccentricity  of  the  orbit.     The  favorable  oppo- 
sitions, which  come  when  the  planet  is  near  its   perihelion,   occur 
about  every  fifteen  years.     The  last  was  in  August,  1892.     At  such 
times  Mars  is  a  brilliant  object,  shining  with  a  fiery  red   light,  and 
fairly  rivalling  Jupiter  in  splendor.     It  is  then  more  than  fifty  times 
as  bright  as  when  faintest,  at  conjunction.     When  not  near  opposi- 
tion, it  might  frequently  be  mistaken  by  an  unpractised  eye  for  one 
of  the  brightest  of  the  fixed  stars.     Its  motion  among  them  would 
lead  to  its  speedy  identification.     No  other  planet  looks  red,  except 
when  near  the  horizon. 

212.  Phases:  Appearance  in  a  Small  Telescope.  —  When   at  oppo- 
sition the  planet's  disk  looks  round,  as  seen  with  a  small  telescope, 
but  at  quadrature  it  is  plainly  gibbous.     For  at  that  time  we  are  not 
in  line  between  Mars  and   the   sun,   and   so  do  not  see  all  of  its 
illuminated  hemisphere.    (See  exercise  21  at  the  end  of  this  chapter.) 
Besides  the  phase,  an  eye  armed  with  a  small  telescope  (three  or 
four  inches  in  aperture)  sees  at  opposition  that  the  surface  is  not  all 
red,  but  bears  certain  darker  markings,  generally  thought  to  be  of 
an  olive-green  hue.     At  times  a  small  white  spot  may  be  seen   at 
one  of  the  poles.      The  dark  spots  are  supposed  to  be  due  to  the 
presence  of  water ;   the  white  polar  spot  suggests  snow. 

213.  The  Polar  Caps.  —  The    caps   are   generally  believed  to  be 
composed  of  snow  and  ice,  not  only  because  of  their  white  appear- 
ance and  their  situation,  but  also  because  the  northern  one  dimin- 
ishes in  size  during  summer  time  in  Mars's  northern  hemisphere,  and 
increases  during  the  winter.     The  southern  cap  behaves  in  a  similar 
fashion.     During  the   opposition   of   1892,  the  southern  polar  cap 
seemed  to  diminish  with  great  rapidity.     Its  area  was  estimated  to 
lose  1,500000  square  miles  in  a  month.     First  a  dark  spot  was  seen 
in  the  snow :   this  spot  gradually  enlarged,  splitting  the  cap  into  two 
parts,  each  of  which  melted  away  at  an  astonishing  rate.     On  several 


MARS. 


occasions  white   spots,  apparently  detached   snowfields,  were  seen 
lying  close  to  the  main  cap. 

In  1894  a  similar  melting  took  place:  in  May  hundreds  of  square 
miles  of  the  polar  cap  disappeared  daily.  During  the  melting  a 
dark  band  surrounded  the  cap,  keeping  at  its  edge  continually,  as 
would  be  expected  if  the  snow  and  ice  were  turning  into  water.  In 
October  the  cap  had  become  so  small  that  it  was  seen  with  the 


Fig.  133.  —  MARS:  DRAWN  BY  BARNARD. 

Lick  telescope  only ;   the  remnant  of  it  seemed  to  be  almost  hidden 
by  some  overhanging  veil. 

Fig.  133  shows  the  planet  as  it  appeared  to  Dr.  E.  E.  Barnard1 
with  the  Lick  36-inch  telescope  on  August  19,  1892.  The  polar 
cap  was  then  only  one  third  as  large  in  area  as  in  June  of  the  same 
year. 

1  Now  of  the  Yerkes  Observatory. 


152  DESCRIPTIVE    ASTRONOMY. 

214.  Seas.  —  The  dark  areas,  if  really  seas,  as  generally  supposed, 
are  not  as   permanent  in   form  as  the  oceans  on  the  earth.     The 
permanent  water  area  has  been  estimated  at  about  500,000   square 
miles,  which  is  only  half  as  great  as  that  of  the  Mediterranean  Sea. 
When  the   polar  cap  melted  in   the  summer  of  1892,  a  portion  of 
the  region  between  it  and  one  of  the  seas  became  dark,  and  the  sea 
apparently  increased  in  size.     While  we  cannot  be  confident  about 
the  cause  of  such   changes,  it  has  been  suggested  that  the  water 
produced  by  the  quick  melting  of  the  polar  cap  flowed  across  the 
land  to  a  sea,  increasing  its  size  temporarily. 

Though  some  of  the  dark  portions  of  the  planet's  surface  are 
probably  bodies  of  water,  there  is  reason  to  believe  that  much  of 
the  dusky  area  is  not  permanently  covered  with  water.  For 
canals  (to  be  described  later)  have  been  seen  in  these  "  seas," 
and  many  different  shades  of  color  exist  there :  the  same  regions 
hav,e  different  tints  at  different  times.  Sometimes  a  vast  amount 
of  detail  is  perceived,  which  would  scarcely  be  found  upon  a  water 
surface.1 

215.  Continents  and  Islands.  —  The  reddish  portion  of  the  planet's 
disk  is  supposed  to  be  dry  land.     But  these  hypothetical  continents 
are  not  secure  in  their  boundary  lines.     Disappearances  of  portions 
of  them  have  been  noted ;   they  seem  to  be  inundated  by  the  waters 
of  neighboring  seas.     Dr.  S.  P.  Langley,  in  his  work  entitled  "  The 
New   Astronomy,"    states    that   Lockyer   Land    is   sometimes    seen 
white,  as  if  covered  with  ice ;  further,  that  Hall  Island  has  this  white 
appearance  so  frequently  as  to  suggest  the  idea  that  some  mountain 
or  table-land  on   it   rises  into  the  region  of  perpetual  snow.     The 
changes  on  the  surface  of  Mars  during  the  opposition  of  1892  were 
so  noteworthy  that  Dr.  E.  E.  Barnard  was  led  to  write  as  follows : 

1  Dr.  Barnard  describes  some  of  his  observations  with  the  Lick  telescope  in  1894 
as  follows :  — 

"  Under  the  best  conditions  these  dark  regions,  which  are  always  shown  with 
smaller  telescopes  of  nearly  uniform  shade,  broke  up  into  a  vast  amount  of  very  fine 
details.  I  hardly  know  how  to  describe  the  appearance  of  these  '  seas '  under  these 
conditions.  To  those,  however,  who  have  looked  down  upon  a  mountainous  country 
from  a  considerable  elevation,  perhaps  some  conception  of  the  appearance  presented 
by  these  dark  regions  may  be  had.  From  what  I  know  of  the  appearance  of  the  coun- 
try about  Mount  Hamilton,  as  seen  from  the  observatory,  I  can  imagine  that,  as  viewed 
from  a  very  great  elevation,  this  region,  broken  by  canyon  and  slope  and  ridge,  would 
look  just  like  the  surface  of  these  Martian  'seas.'  " 


MARS.  153 

"  These  striking  changes  are  enough  to  make  us  pause  and  question 
whether  what  we  see  before  us  in  the  heavens  is  really  another 
world  like  our  own,  with  relatively  fixed  oceans  and  continents, 
or  whether  it  is  not  a  world  like  our  own  in  its  younger  days, 
when  continents  were  shifting  and  oceans  changing,  before  the 
surface  of  the  earth  became  firm  and  fixed  by  the  process  of 
cooling." 

Some  of  the   apparent  changes   in  the  forms   of  the   continents 
have  been  ascribed  to  the  spread  of  vegetation  along  their  borders. 

216.  Clouds.  -  -  Transient   spots,    having   the    general    aspect   of 
cloud  masses,  have  been  observed.     Sometimes  they  are  small  and 
of   tolerably  definite    outline,  but   usually  they  are    diffuse   and   of 
large  extent.     They  have  also  appeared  as  long  streaks  projecting  a 
trifle  beyond  the  planet's    limb.      At  times  portions  of  the    land- 
scape have  been  so  obscured  as  to  give  rise  to  the  theory  that  the 
obscuration  was  caused  by  a  passing  cloud.     But  many  details  of 
the  Martian  landscape  are  usually  seen  so  plainly  that  clouds  must  be 
considered  as  rarities.     Their  comparative  absence  would  naturally 
follow  from  the  small  proportion  of  water  surface. 

217.  Atmosphere.  —  Were   the    atmosphere    dense,    like   that    of 
Venus,  we  should  never  have  discovered  the  mass  of  topographical 
detail  now  known.     The  rarity  of  Mars's  atmosphere  has  been  ac- 
counted for  by  the  moderate  size  of  the  planet,  and  the  weakness  of 
the  force  of  gravity  at  its  surface.     Could  a  rifle  ball   be  shot  up- 
ward with  a  velocity  of  3.5   miles  a  second,  it  would,  unless  checked 
by  atmospheric  resistance,  leave  the  planet  never   to  return.     The 
speed  of  molecules  of  hydrogen,  in   their  incessant   vibration,  may 
considerably  exceed  this,  so  that  free  hydrogen  is  not  to  be  looked 
for  as  a  constituent  of  Mars's  atmosphere.     The  best  spectroscopic 
observations  indicate  that  the  atmosphere  of  Mars  exerts  no  meas- 
urable absorptive  effect  upon  the  sunlight  which  strikes  through  it, 
and  is  reflected  back  to  us.     We  are  therefore  ignorant  of  its  com- 
position, and  can  only  say  that,  if  it  be  similar  to  that  of  our  air,  its 
average  density  can  scarcely  be  one  fourth  as  great. 

218.  Description  of  the  Canals. -^- In   1877   Schiaparelli  discovered 
several  of  the  markings  which  are  commonly  called  "  Schiaparelli's 
canals."     A  few  had  been  seen  previously.     Many  more  have  since 
been  found  by  him  and  by  others.     The  map  of  Mars  made  at  the 


154  DESCRIPTIVE    ASTRONOMY. 

Lowell  observatory1  in  1894,  exhibits  a  bewildering  network  of 
canals,  connecting  small  dark  spots  scattered  over  the  surface.  Not 
infrequently  half  a  dozen  canals  radiate  from  a  single  spot,  going 
straight  to  other  spots.  Most  of  the  canals  choose  the  shortest 
path  from  one  spot  to  another:  a  few  are  curved.  Some  do  not 
run  from  one  small  dark  spot  to  another,  but  connect  large  dark 
areas,  or  go  from  a  small  spot  to  a  large  area,  or  occasionally  con- 
nect two  other  canals.  The  small  spots  are  less  than  1 50  miles  in 
diameter.  The  length  of  the  canals  ranges  from  a  few  hundred  to 
3,500  miles.  Their  average  breadth  is  30  miles.  The  most  myste- 
rious fact  about  them  is  that  they  become  double  at  times,  the 
two  new  canals  being  about  200  miles  apart,  and  veritable  twins. 
Schiaparelli  thinks  that  the  doubling  may  be  periodical,  and  con- 
nected in  some  way  with  the  planet's  seasons. 

219.  Explanations  of  the  Canals.  — The  canals  have  naturally  been 
supposed  to  be  water-ways.     When  a  polar  cap  melts,  the  canals 
in  the  neighborhood   become  darker  and  wider,  and  remain    dark 
until    the    snow   stops    melting.      Then    the   width    of   the    canals 
diminishes.     These  appearances  have  led  Schiaparelli  to  the  con- 
clusion   that   the   canals   are    natural    furrows,   through    which    the 
water  is  carried  from  the  poles  equatorward.     Mr.  Percival  Lowell 
advocates  the  theory  that  the  canals  are  strips  of  vegetation,  which 
are  watered  by  canals  too  small  to  be  visible  to  us.     A  small  spot 
at  the  junction  of  several  canals  is  an  oasis,  according  to  this  view. 
No    satisfactory    explanation    of   the    doubling    of   the    canals    has 
been   given.     The   majority  of  astronomers,  while   freely  admitting 
the   existence   of  the    markings   called   canals,   are   inclined    to    be 
conservative  with  reference  to  any  explanation  of  their  nature.     It 
has  been  aptly  said   that  it  is  better  not  to  know  so  much,  than  to 
know  so  many  things  that  are  not  so. 

220.  Colors.  —  Orange  and  grayish  green  are  the  prevailing  col- 
ors, outside  of  the  polar  caps.      But  various  colors  have  been  seen 
in   different  spots.      The   same   spot  has  been  of  different  hues  at 
different  times,  though  the  utmost  care  was  taken  to  avoid  optical 
illusions.     Light  greens    and  bright  greens   have   been   seen  often. 
At  times   places  supposed  to  be  bodies   of  water  have  exchanged 

1  A  temporary  observatory  set  up  at  Flagstaff,  Arizona,  by  Mr.  Percival  Lowell, 
of  Boston. 


MARS. 


^-..^    V- /--«MH; 5j    ^ 

*\"  >*H\>  5^a    -  *'f^3k  ^ 

'\  HK^  ii       r#i  r.»c^>4<!> 
iVtf^^ff/ls'^x 


1 — ^^r—  -  rr    s«  o 
f  :     >^      t-Ti^ 


J^T**  grr<v  7  li^^to^l^ 

-::i   o,    ii  ^CTM> 


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ax  --^ 

\        1  3= v  '< 


156  DESCRIPTIVE    ASTRONOMY. 

their  ordinary  color  for  dark  blue.  Gray  and  yellow  tints  are  of 
common  occurrence.  Even  so  extraordinary  a  color  as  violet-lake 
was  once  perceived.  •  Some  of  these  colors  may  be  explained  as 
due  to  haziness  or  partial  cloudiness.  Perhaps  some  are  due  to 
the  presence  of  vegetation. 

221.  Satellites.  —  Mars  is  attended  by  two  moons,  discovered  by 
Prof.  Hall 1  in  August,  1877.    Their  names  are  Deimos  and  Phobos.2 
The  distance  of  Deimos  from  the  planet's  centre  is   14,600  miles; 
it  completes  a  revolution  about  Mars  in  30  h.  18  m. 

Phobos  is  at  a  distance  of  but  5,800  miles  and  takes  only  7  h. 
39  m.  for  one  revolution.  The  time  of  rotation  of  Mars  being 
over  24  hours,  Phobos  by  reason  of  its  rapid  motion  rises  in  the 
west  and  sets  in  the  east,  as  our  moon  would  if  its  orbital  motion 
were  swift  enough. 

These  moons  are  so  minute  that  it  is  not  possible  to  measure 
their  diameters  directly ;  but  by  measures  of  the  amount  of  light 
that  they  give,  Prof.  Pickering  3  has  concluded  that  the  inner  one 
is  7  miles  in  diameter,  the  outer  one  5  or  6.  Estimates  made  at  the 
Lowell  observatory  give  a  diameter  of  36  miles  for  Phobos  and  10 
miles  for  Deimos.  Their  discovery  was  as  great  a  feat  of  telescopic 
vision  as  for  a  man  in  Boston  to  see  a  tennis  ball  at  Philadelphia. 

222.  Habitability.  —  If  we  have  simply  to  answer  the  question, 
"  Would  a  man,  as  constituted  at  present,  if  transported  to   Mars, 
find    it   possible    to    exist   there?"    the    most   probable  answer  is, 
"  No."      While  one  must  not  be  dogmatic,   it   may  be   said,  with 
some  assurance,   that  the   man  would  gasp   a   few  times   and  die. 
However,  it  is  conceivable  that  manlike  beings  might  find  a  home 
there.      Plans    for   communication   with    the    supposititious    inhab- 
itants of  Mars  by  means  of  huge  signals  displayed  in  deserts  or  on 
table-lands,  or  by  gigantic  combinations  of  electric  lights,  are  little 
better  than  phantasies  of  a  disordered  imagination.     If  some  enter- 

1  Prof.  Asaph  Hall,  formerly  of  the  U.  S.  Naval  Observatory,  Washington,  D.  C. 

2  These  names  were  given  by  Homer  to  the  steeds  which  drew  the  chariot  of  the 
god  of  war.     In  one  passage  in  the  fifteenth  book  of  the  Iliad  they  are  personified, 
and  refer  to  the  attendants  of  Mars.    Bryant's  translation  is  :  — 

"  He  spake,  and  summoned  Fear  and  Flight  to  yoke 
His  steeds  and  put  his  glorious  armor  on." 

3  Edward  C.  Pickering,  Director  of  the  Harvard  College  Observatory. 


MARS.  157 

prising  and  athletic  individual  on  Mars  should  wave  a  flag  as  large 
as  the  State  of  New  York,  terrestrial  astronomers  might  notice 
his  greeting. 

There  have  been  curious  anticipations  of  the  discovery  of  the 
moons  of  Mars.1 

Kepler,  in  a  letter  written  after  the  discovery  of  four  satellites  of 
Jupiter  by  Galileo,  said :  "  I  am  so  far  from  disbelieving  the 
existence  of  the  four  circumjovial  planets,  that  I  long  for  a  tele- 
scope to  anticipate  you,  if  possible,  in  discovering  two  around 
Mars,  as  the  proportion  seems  to  require,  six  or  eight  around 
Saturn,  and  perhaps  one  each  around  Mercury  and  Venus." 

Swift,  in  his  satire,  "  The  Travels  of  Mr.  Lemuel  Gulliver,"  puts 
the  following  language  into  the  mouth  of  Gulliver,  who  had  arrived 
among  the  inhabitants  of  Laputa,  a  flying  island :  — 

"The  knowledge  I  had  in  mathematics  gave  me  great  assistance  in 
acquiring  their  phraseology,  which  depended  much  upon  that  science,  and 
music  :  and  in  the  latter  I  was  not  unskilled.  Their  ideas  were  perpetu- 
ally conversant  in  lines  and  figures.  If  they  would,  for  example,  praise 
the  beauty  of  a  woman,  or  any  other  animal,  they  describe  it  by  rhombs, 
circles,  parallelograms,  ellipses,  and  other  geometrical  terms,  or  by  words 
of  art  drawn  from  music,  needless  here  to  repeat.  .  .  .  They  spend  the 
greatest  part  of  their  lives  in  observing  the  celestial  bodies,  which  they  do 
by  the  assistance  of  glasses  far  excelling  ours  in  goodness.  For  although 
their  largest  telescopes  do  not  exceed  three  feet,  they  magnify  much  more 
than  those  of  a  hundred  with  us,  and  show  the  stars  with  greater  clearness. 
This  advantage  has  enabled  them  to  extend  their  discoveries  much  farther 
than  our  astronomers  in  Europe ;  for  they  have  made  a  catalogue  of  ten 
thousand  fixed  stars,  whereas  the  largest  of  ours  do  not  contain  above  one 
third  of  that  number.  They  have  likewise  discovered  two  lesser  stars  or 
satellites  which  revolve  about  Mars :  whereof  the  innermost  is  distant  from 
the  centre  of  the  primary  planet  exactly  three  of  his  diameters,  and  the 
outermost  five  ;  the  former  revolves  in  the  space  of  ten  hours,  and  the 
latter  in  twenty-one  and  a  half;  so  that  the  squares  of  the  periodical  times 
are  very  nearly  in  the  same  proportion  with  the  cubes  of  their  distances 
from  the  centre  of  Mars ;  which  evidently  shows  them  to  be  governed  by 
the  same  law  of  gravitation  that  influences  the  other  heavenly  bodies." 

1  The  following  interesting  information  is  derived  from  Professor  Hall's  monograph 
on  the  satellites. 


158  DESCRIPTIVE    ASTRONOMY. 

Voltaire  represents,  in  one  of  his  works,  that  Micromegas,  an 
inhabitant  of  Sirius  who  visited  our  system,  discovered  that  Mars 
had  two  moons,  which  made  perpetual  compensation  for  the  lack  of 
the  brilliant  sunlight  which  we  enjoy. 

THE  ASTEROIDS,  OR  MINOR  PLANETS. 

223.  Bode's  Law.  —  In   1772   the  astronomer  Bode  brought  into 
prominence  a  relation  between  the  distances  from  the  sun  of  the 
then  known  planets.     This  relation  had  been  discovered  some  years 
previously  by  Titius,  but  it  is  commonly  called  Bode's  Law,  and  is 
found  as  follows.     The  series  o,  3,  6,  12,  etc.,  in  which  each  number 
except  the  first  is  double  the  preceding  one,  is  written.     To  each 
term  4  is  added. 

03  6         12         24        48  96         192 

4  7         10         16         28         52         100         196 

The  resulting  numbers  represent  fairly  the  relative  distances. 
Taking  the  earth's  distance  as  ten,  the  real  distances  are  given 
below. 

Mercury     .     .     .  3.9  Jupiter  .  .  .  52.0 

Venus    '   V    .     .  7.2  Saturn  .  .  .  95.4 

Earth    ....  10.0  Uranus  .  .  .  191.8 

Mars      .     .     .     .  15.2 

The  number  28  in  the  scheme  of  Titius  corresponded  to  no  known 
planet.  Astronomers  generally  became  imbued  with  the  notion  that 
there  was  a  planet  to  be  discovered,  which  would  fill  the  gap. 

Neptune  was  then  unknown :  its  distance  does  not  conform  to  the 
law,  being  only  300.5. 

224.  Discovery  of  the  First  Minor  Planet.  —  After  Uranus  was  dis- 
covered in  1781,  and  its  distance  had  been  found  to  conform  to  the 
law  of  Titius,  an  association  of  astronomers  was  formed,  to  hunt  for 
the  missing  planet  between  Mars  and  Jupiter.     But  the  honor  of  the 
discovery  was  reserved  for  Piazzi,  of  Palermo,  who  was  not  a  member 
of  the  association,  but  was  engaged  upon  a  star  catalogue.     It  was 
his  habit   to  observe  the  right   ascension  and    declination  of  each 
star  on  several  different  nights,  that  he  might  determine  its  place 
accurately. 


THE    ASTEROIDS,    OR    MINOR    PLANETS.  159 

Upon  the  evening  of  the  first  day  of  the  nineteenth  century,  he 
observed  a  list  of  stars,  one  of  which  was  destined  to  bring  him 
renown.  On  January  2d,  3d,  and  4th  he  reobserved  the  same  list, 
and  upon  comparing  his  observations,  discovered  that  the  thirteenth 
star  on  the  list  changed  its  position  from  night  to  night.  Here 
at  last  was  a  planet,  but  was  it  the  one  sought?  For  six  weeks 
he  observed  it  upon  every  opportunity,  and  then  he  fell  ill :  when 
he  recovered,  the  planet  was  so  near  to  the  sun  that  it  could  no 
longer  be  found. 

225.  Gauss  computes  the  Orbit.  —  Here  was  a  dilemma.    The  news 
reached  Germany  in  the  early  spring,  and  Gauss,  of  Gottingen,  then 
a  rising  young  mathematician,  set  himself  at  the  task  of  finding  some 
method  of  computing  its  orbit.    Finally  he  discovered  the  now  classic 
method  of  computing  a  planet's  orbit,  when  its  right  ascension  and 
declination  are  known  at  three  different  dates.     He  applied  the  new 
method  to  Piazzi's  observations,  and  predicted  the  future  place  of 
the  planet;   at   last  it  was    bound  by   the  chains  of  mathematical 
analysis,  more  ethereal  than  a  spider's  web,  but  stronger  than  bronze. 
The  association  was  already  hard  on  the  track  of  the  wanderer,  and 
when  Gauss's  results  reached  them,  one  of  them  speedily  rediscov- 
ered it,  on  the  last  day  of  the  year.     Its  distance  from  the  sun  was 
in  fair  agreement  with  the  law  of  Titius,  being  25.7  instead  of  28. 
It  received  the  name  Ceres. 

226.  Further  Discoveries.  —  In  1802,  1804,  and   1807,  Pallas,  Juno, 
and  Vesta  were  found.     Astronomers  were  not  again  successful  in 
the  quest  until  1845,  when  Astraea  was  brought  to  light.     Soon  after- 
ward the  pace  of  discovery  quickened,  and  now  the  asteroid  hunters 
find  them  with  almost  embarrassing  rapidity.     The  family  of  these 
little  strangers  is  so  large,  and  increasing  so  rapidly,  that  the  problem 
of  taking  care  of  them  is  becoming  a  very  serious  one.     The  orbit 
of  each  new  one  has  to  be  computed,  and  its  place  in  the  sky  (right 
ascension  and  declination)  calculated  from  year  to  year,  so  that  fresh 
observations  can  be  taken,  and  a  more  accurate  orbit  figured  out. 

227.  Methods  of  Search.  —  Almost  all  of  these  bodies  are  so  small 
that  they  look  like  fixed  stars,  and  can  be  detected  only  by  their 
motion.     An   observer  makes   a  chart  of  a   certain   region  of  the 
sky,   containing    all  the    stars  visible  there   through   his   telescope. 
Then  night  after  night  he  compares  the  heavens  with  his  chart.     If  a 


i6o 


DESCRIPTIVE    ASTRONOMY. 


SUN 


star-like  object  not  on  the  chart  is  seen,  its  position  is  carefully  noted. 
In  a  few  hours  its  motion  will  betray  it,  if  it  be  a  minor  planet. 

This  process  is  quite  laborious,  and  is  now  little  used  because 
photography  has  entered  the  field.  A  picture  of  a  certain  region 
of  the  sky  is  taken,  the  plate  being  exposed  for  a  couple  of  hours. 
While  the  images  of  stars  on  the  plate  are  circular,  the  image  of  the 
planetoid  is  a  short  streak,  caused  by  its  motion. 

228.  Orbits:    Distances:   Periods.  —  The  orbits  of  the  planetoids 
are    more    eccentric  than    those    of   the  major  planets.     The  orbit 
planes  have  greater  inclinations  to  the  ecliptic,  the  orbit  of  Pallas 
being  inclined  35°.     The  mean  distances  vary  from   198,000000  to 
400,000000  miles,  the  pe- 
riods from  three  years  to 

nine. 

The  computations  of 
the  orbits,  as  well  as  of 
the  annual  ephemerides> 
giving  the  right  ascen- 
sion and  declination  of 
the  minor  planets,  are 
made  chiefly  by  German 
computers. 

The    Berliner    Jahr- 

buch,     issued      from      the  Fig.  135.- THE  ZONE  OF  ASTEROIDS. 

Imperial  Observatory  at  Kiel,  Prussia,  contains  the  results  of  their 
work. 

The  late  Prof.  Watson J  left  a  fund  to  pay  for  the  twenty-two 
which  he  discovered,  intrusting  it  to  the  American  National  Academy 
of  Sciences. 

229.  Designations.  —  Each  asteroid  has  a  number,  which  is  usually 
printed  thus,  ®,  the  numbers  being  given  in  the  order  of  discov- 
ery.    Names   are  also  given,  chosen  chiefly  from  those  of  female 
divinities   in  classical   mythology.     The  number  of  these  is  about 
exhausted.     Some  of  the  unfortunate  planetoids  have  been  afflicted 
with   such    names    as    Xantippe,    Vindobona,    and    Sophia,    to    say 
nothing  of  Walpurga  and   Chicago.     The  asteroid   last  mentioned 

1  James  C.  Watson,  Director  of  the  Observatory  at  Ann  Arbor,  Mich.,  and  later  of  the 
Washburn  Observatory  at  Madison,  Wis.,  author  of  Watson's  Theoretical  Astronomy. 


THE    ASTEROIDS,    OR    MINOR    PLANETS.  l6l 

was  named    by  the  Astronomical  Congress  which  met  at  Chicago 
during  the  World's  Fair. 

230.  Number  and  Size.  —  The  number  is  now  (1896)   over  400. 
Most  of  them  are   of  quite    insignificant   dimensions.     Vesta,   the 
brightest  one,  is  250  miles  in  diameter.     At  opposition  it  is  barely 
visible  to  a  good  eye.     Ceres,  the  largest,  is  nearly  500  miles  in 
diameter.     The  majority  are  less  than  fifty  miles  in  diameter.     Most 
of  the  faint  ones  now  being  discovered  have  diameters  of  only  about 
ten  miles.     Compared  with  the  earth  they  are  as  flour-dust  to  a  foot- 
ball.    Half  a  billion  of  them  compacted  together  might  equal  the 
earth  in  bulk. 

231.  Atmosphere :  Gravity.  —  The  bodies  are  so  small   that   they 
probably  have  very  rare  atmospheres.     There  are  no  reliable  obser- 
vations  bearing  on  this  point.     On  the  assumption  that   they  are 
as  dense  as  the  earth,  the  force  of  gravity  at  the  surface  of  one 
of  the  small  ones  is  about  one  thousandth  part1  of  that  which  we 
daily  contend  with.     A  sharply  batted  base-ball  would  leave  the 
planet;    in  a  jumping  match  the  spectators  could  eat  lunch  while 
waiting  for  the  contestants  to  come  back  to  terra  firma. 

232.  Origin.  —  One  view  is  that  they  have  been  formed  from  a 
larger  body  by  a  series  of  explosions.     A  single  explosion  would 
not  account  for  them,  for  the  fragments,  after  coursing  about  the 
sun  would  all  return  to  the  point  where  the  explosion   occurred. 
After  the  lapse  of  ages  their  orbits  would  be  considerably  modified 
by  the  attractions  of  the  major  planets,  especially  by  that  of  Jupiter. 
But  yet  the  theory  of  a  single  explosion  is  considered  untenable 
by  those  mathematicians  who  have  given  particular  attention  to  the 
matter.     The  present  tendency  of  scientific  opinion  is  to   discard 
catastrophes,   and   to    believe    in    an    orderly  evolution    manifested 
throughout  the  universe.     According  to  the  nebular  hypothesis,  the 
material  composing  the  minor  planets  was  once  collected  in  a  ring 
surrounding  the  sun.     The  ring  in  condensing  formed  many  planets 
instead  of  one:   the  cause  of  the  disruption  of  the  ring  may  have 
been  the  powerful  attraction  of  Jupiter. 

1  The  principles  of  mechanics  show  that,  for  spheres  of  equal  densities  the  force  of 
gravity  at  their  surfaces  varies  as  their  diameters.  Thus,  if  two  leaden  spheres  were 
respectively  one  foot  and  ten  feet  in  diameter,  a  grain  of  sand  lying  on  the  surface  of 
the  latter  would  be  attracted  by  it  ten  times  as  strongly  as  an  equal  grain  on  the 
surface  of  the  former. 

II 


1 62  DESCRIPTIVE    ASTRONOMY, 

EXERCISES. 

233.  i.  If  the  axis  of  Mercury  is  perpendicular  to  the  plane  of  its 
orbit,  and  the  planet  rotates  on  its  axis  at  a  uniform  rate  from  west 
to  east  once  in  every  revolution,  are  there  alternations  of  day  and 
night  at  a  point  on  the  surface  directly  between  the  centres  of  the 
sun  and  the  planet? 

2.  In  the  case  above,  would  the  sun  shine  on  more  than  half  of 
Mercury's    surface  in   88  days,  considering  the  eccentricity  of  its 
orbit? 

3.  On  account  of  the  relative  sizes  of  the  sun  and  Mercury,  does 
the  former,  at  each  instant,  illuminate  more  than  a  hemisphere  of 
the  latter's  surface? 

4.  Why  do  Mercury  and  Venus  look  black  during  transit? 

5.  Mercury  at  times  attains  an  elongation  of  28°  from  the  sun. 
If  it  is  at  eastern  elongation  about  the  last  of  March,  its  orbit  being 
roughly  coincident  with  the  ecliptic,  will  it  be  above  the  celestial 
equator,  or  below?     [In  answering  this,  first  fix  in  mind  the  position 
of  the  sun  in  the  ecliptic.] 

6.  Should  a  planet,  in  order  to  be  seen  most  advantageously 
from  your  home,  be  above  or  below  the  equator? 

7.  If  Mercury  be  near  its  western  elongation  on  April  I,  will  it  be 
in  a  favorable  position  to  be  seen  from  your  home? 

8.  If  Mercury  be  near  its  eastern  elongation  when  school  opens 
in  the  fall,  will  it  be  in  a  favorable  position  for  observation  by  you  ? 

9.  If  you  were  searching  for  Mercury  with  the  naked  eye,  would 
it  be  more  convenient  to  have  it  at  eastern  or  western  elongation, 
considering  only  the  time  of  night  at  which  you  would  look  for  it? 

10.  When  Venus  is  morning  star,  is  it  east  of  the  sun,  or  west? 

11.  Draw  two  concentric  circles  of  the  proper  relative  sizes   to 
represent   the    orbits    of    Venus    and    the    earth.     Locate   the   two 
planets  upon  them,  so  that  Venus  shall  be  at  its  greatest  elongation. 
Then  measure  the  angle  of  elongation  with  a  protractor. 

12.  What  phase  has  Venus  when  at  elongation? 

13.  Is  Venus  gibbous  between  its  superior  conjunction  and  its 
greatest  eastern  elongation  ? 

14.  Is  Venus   gibbous  between  its   greatest  western  elongation 
and  superior  conjunction? 


MERCURY,    MARS,    VENUS,    THE    ASTEROIDS.  163 

15.  Just  before  inferior  conjunction  do  the  cusps  of  the  crescent 
Venus  point  toward  the  sun? 

1 6.  Find  from  the  Nautical  Almanac  when  Mercury  and  Venus 
next  reach  elongation. 

17.  If  you  have  a  telescope,  make  the  following  test  of  it  and 
your  eye.     Make  on  white  paper  a  drawing  like  Fig.   136.     Look 

at  it  with  the  telescope,  and  estimate  the 
width  of  the  black  drop  as  compared  with 
the  diameter  of  the  black  circle  representing 
the  planet.  It  is  well  to  perform  the  experi- 
ment out  of  doors,  with  your  back  to  the  sun, 
so  that  the  paper  may  be  well  illuminated. 
l  6  1 8.  Why  is  not  a  ring  of  light  seen 

around  Venus,  when  it  is  in  transit? 

19.  The  volumes  of  two  spheres  are  to  each  other  as  the  cubes 
of  their  diameters.     The  earth  is  how  many  times  as  large  as  Mars? 

20.  Assume  that  the  diameter  of  the  earth  is  8,000  miles,  and 
that  of  some  asteroid  is  10  miles.     The  earth  is  how  many  times  as 
large  as  the  asteroid? 

21.  Draw  two  concentric  circles  of  the  proper  relative  diameters 
to  represent  the  orbits  of  the  earth  and  Mars.     (The  representation 
will  not  be  very  accurate,  because  Mars's  orbit  is  quite  eccentric.) 
Mark  a  point  on  the  inner  circle  for  the  earth,  and  another  on  the 
outer  to  represent  the  position  of  the  centre  of  Mars,  when  in  quad- 
rature.    Around  this  centre  draw  a  little  circle,   to  represent   the 
planet  itself,  on  an  exaggerated  scale.     Shade  one  half  of  this  little 
circle,  so  that  it  will  represent  the    unilluminated    hemisphere    of 
Mars.     How  does  the  picture  show  that  Mars,  as  seen  from  the 
earth,  would  be  gibbous? 

22.  Does  any  planet,  as  seen  from  the  sun,  exhibit  phases? 

23.  Deimos  and  Phobos  move  in  the   plane  of  Mars's  equator. 
Could  an  observer  at  a  pole  of  the  planet  see  them? 

24.  Compute  the  area  of  the  surface  of  Deimos,  assuming  it  to 
be  a  sphere  six  miles  in  diameter. 

25.  Is  Bode's  Law  anything  more  than  a  chance  coincidence? 

26.  The  first  day  of  this  century  was  January  1st  in  what  year? 

27.  Why  does  the  small  size  of  an  asteroid  militate  against  its 
having  a  dense  atmosphere? 


164  DESCRIPTIVE    ASTRONOMY. 

CHAPTER  X. 

JUPITER,  SATURN,  URANUS,  NEPTUNE. 

"  Some  displaying 

Enormous  liquid  plains,  and  some  begirt 
With  luminous  belts,  and  floating  moons,  which  took, 
Like  them,  the  features  of  fair  earth." 

BYRON. 

234.  The  Outer  Group. — We  come  now  to  the  consideration  of 
the  outer  group  of  planets,  in  comparison  with  which  the  planets 
before  considered  are  but  pygmies. 

JUPITER,    y.. 

235.  Distance  and  Diameter.  —  The  mean  distance  of  Jupiter  from 
the   sun  is  483,000000   miles;   this  is  five  and   one  fifth  times  the 
distance  of  the   earth.     Its  mean   diameter   is  88,000  miles.     The 
planets    heretofore   considered   are    nearly  spherical,    but   Jupiter's 
disk  as  seen  in  a  telescope  is  a  marked  oval,  showing  that  the  polar 
diameter  of  the  planet  is  shorter  than   the   equatorial.1     Jupiter  is 
larger  than  all  the  other  planets  put  together,  being  1,300  times  as 
large  as  the  earth. 

236.  Revolution  and  Rotation.  —  The  sidereal  period  of  this  planet 
is  nearly  twelve   ( n.86)   years.     Despite  its  huge  bulk,  it  rotates 
with    amazing  swiftness,  in   about  9  h.  55m.     The   rotation  period 
cannot  be  obtained  with   exactness,  because,  like  the  sun,  different 
parts  of  the  surface  rotate  in  different  times,  those  near  the  equator 
moving  most  rapidly.     Even  a  particular  feature  of  his  surface  does 
not  have  a  uniform  rotation  time.     The  time  for  the  great  red  spot 
(§  240),  for  example,  slowly  increased  from  year  to  year  during  the 
period  1878-86,  the  total  increase  being  seven  seconds.     Since  1886 
there  has  been  no  change. 

1  The  polar  diameter  is  84,300  miles  and  the  equatorial  89,790  miles.  The  mean 
diameter  is  found  by  adding  the  polar  diameter  to  twice  the  equatorial,  and  dividing 
the  sum  by  3. 


JUPITER.  165 

237.  Appearance  to  the  Naked  Eye.  —  To  the  naked  eye,  Jupiter, 
when  near  opposition,  attains  a  greater  brilliancy  than  any  other 
planet,   except  Venus.     At  all  times,  except  when  too   near  con- 
junction, it  is  much  brighter  than  any  of  the  fixed  stars.     Its  light 
is  white,  with  the  merest  tinge  of  yellow.     Except  when  near  one 
of  its  stationary  points,  its  planetary  nature  can  be  detected  in  less 
than  a  week  by  its  motion  among  the  stars. 

Men  of  extremely  acute  vision,  under  the  fairest  of  skies,  have 
occasionally  seen  one  of  its  satellites  with  the  naked  eye.  One  who 
suspects  that  he  sees  a  satellite  should  move  his  head  from  side  to 
side,  and  see  if  the  suspected  satellite  moves  also ;  if  it  does  not, 
it  may  be  a  satellite  or  a  fixed  star. 

238.  Belts.  —  These  are  readily  discerned  with  a  small  telescope. 
The  surface  seems  banded  by  parallel  belts,  the  most  conspicuous 
ones  being  near  the  planet's  equator.     The  belts  are  dark  colored. 
In  small  instruments  the  dark  belts  appear  of  a  grayish  or  brownish 
cast ;  but  in  larger  ones  a  reddish  hue  is  observed.     At  times  they 
even  appear  pink.     The  belts  are  supposed  to  be  rifts  in  the  clouds, 
through  which  we  look  down  deeper  into  his  atmosphere  than  else- 
where.    The  light  portions  of  the  disk,  as  shown  in  Fig.  137,  re- 
semble masses  of  cumulus  cloud.     The  appearance  of  the  belts  is 
sometimes  reproduced  in  the  terrestrial  clouds  lying  below  the  sum- 
mit of  Mt.  Hamilton,  Cal.,  the  home  of  the  Lick  observatory.     The 
belts  change  their  outlines  somewhat  from  month  to  month. 

239.  Pickering's  Theory  of  the  Belts.  —  Prof.  W.  H.  Pickering,1 
observing  from  a  high  Peruvian  table-land,  where  the  atmospheric 
conditions  are  very  fine,  says  that  the  appearance  of  Jupiter  is  that 
of  a  uniform  white  mass  of  cloud,  overlaid  by  a  thin  gauze  veil  of  a 
brown  material,  not  unlike  our  cirrus  clouds  in  structure.     This  veil 
is  more  dense  in  some  places  than  in  others ;  the  dense  portions, 
by  obscuring  the  white  cloud  beneath,  cause  the   dark  belts.     The 
well  known  white  spots  are  thought  to  be  due  to  round  or  elliptical 
holes  in  the  cirrus  layer,  through  which  we    see  the  white  surface 
below.     He  says :   "  In  short,  it  appears  that,  were  it  not  for  this 
insignificant  light  gauzy  veil  of  brown  cloud,  we  should   find   the 
surface  of  Jupiter,  like  that  of  most  of  the  other  planets  in  the  solar 
system,  almost  a  perfect  blank.     This  gauzy  structure  must  float  in 

1  Of  the  Harvard  College  Observatory. 


i66 


DESCRIPTIVE    ASTRONOMY. 


/,  Fig.  137. — JUPITER,  AS  SEEN  WITH  THE  LICK  TELESCOPE:  DRAWN  BY  KEELER 


JUPITER.  167 

a  nearly  transparent  atmosphere,  surrounding  and  rising  above  it ; 
it  is  this  atmosphere  which  causes  the  absorption,  and  which  almost 
completely  obscures  the  belts  at  the  limb  of  the  planet." 

240.  The  Great  Red  Spot. — This  was  discovered   in    1878;   it  is 
30,000  miles  long  and  7,000  broad.     It  has  had  various  degrees  of 
brightness   in   different  years,  being  almost   invisible   in   1883    and 
1884,  and  again  in  1892.     It  is  no  longer  a  conspicuous  object,  even 
in  large  telescopes.     The  spot  is  thought  to  be  a  rift  in  the  clouds, 
similar  in  color  to  the  belts,  though  usually  more  vivid.     Its   cause 
is  a  mere  matter  of  conjecture.     Perhaps  it  is  due  to  some  terrific 
ebullition  in  the  depths  of  the  planet,  which   causes  heated  vapors 
to  arise  and  clear  away  the  clouds  which  would  otherwise  be  above 
it.     Its  remarkable  persistency  of  form,  and  its  movement  (§  236) 
are  against  this  supposition. 

241.  Smaller  Spots.  —  These  have  been  seen   frequently  of  late 
years,  and    are   chiefly  either  white    or   black.     They  are    usually 
round  or  elliptical ;   their  average  size  is  about  that  of  our  moon. 
Perhaps  they  are  cloud  masses  which  are  above  the   general  level, 
and  therefore  show  more  plainly.     Their  motions  are  independent, 
as  is  shown  by  the  fact  that  the  rotation  time  of  the  planet,  deter- 
mined by  observations  of  one   spot,  does  not  generally  agree  with 
that  found  by  employing  some  other  spot. 

242.  Atmosphere:  Spectrum.  —  All  the  evidence  points  to  the  con- 
clusion that  the  atmosphere  is  very  extensive.     There  is  no  reason 
to  believe  that  any  permanent  markings  have  ever  been  seen  on  the 
planet,  so  deep  and  dense  is  the  enveloping  atmosphere. 

The  spectroscope  gives  no  certain  evidence  concerning  the  com- 
position of  the  atmosphere,  the  spectrum  of  Jupiter  being  almost 
identical  with  that  of  the  sun.  This  shows  that  the  light  which 
gives  the  spectrum  is  merely  reflected  sunlight.  If  the  light  pene- 
trated to  any  considerable  depth  it  would  suffer  marked  absorption, 
which  would  be  manifested  by  bands  in  the  spectrum.  There  is  one 
large  faint  band  in  the  spectrum,  the  origin  of  which  is  unknown. 

243.  Light  and  Heat.  —  If  Jupiter  were  hot  enough  to  give  out 
light  of  its  own,  its  moons  would  not  suffer  total  eclipse  when  they 
passed  into  its  shadow.     But  a  body  may  be    quite   hot   without 
being   perceptibly  luminous,   as   any  disbeliever  may  discover  by 
experimenting  with  a  poker  recently  withdrawn  from  the  fire.     So 


1 68 


DESCRIPTIVE    ASTRONOMY. 


cloudy  is  the  planet,  so  great  are  the  changes  continually  taking 
place  on  its  surface,  and  so  feeble  is  the  action  of  the  sun  upon  it 
on  account  of  its  great  distance,  that  we  are  compelled  to  believe 
that  the  planet  is  hot.  It  may  be  nearly  self-luminous,  and  has 
been  called  a  semi-sun. 

244.  Physical  Condition.  —  The  average  density  of  the  planet  is 
only  a  third  greater  than  that  of  water.     It  is  therefore  chiefly,  if 
not  entirely,  composed  of  matter  in  a  fluid  state ;   hot  water  may  be 
one  of  its  chief  constituents.     We  are  not  to  regard  it  as  possessing 
a  solid  crust,  like  the  earth,  but  rather  as  being  a  seething  caldron, 
in  which  the  hot  fluids  rise  from  the  interior,  become  cooler,  and 
sink  back,  in  a  ceaseless  round  of  motion. 

245.  The  Major  Satellites. — These  are  four  in  number,  and  are 
designated  in  the  Nautical  Almanac  by  the  Roman  numerals  I,  II, 
III,  and  IV,  according  to  their  distances  from  the  planet,  I   being 


Fig.  138.  —  THE  ORBITS  OF  THE  MAJOR  SATELLITES. 

nearest,  and  IV  the  most  remote.  The  smallest,  II,  is  as  large  as 
the  moon,  and  the  largest,  III,  is  nearly  as  big  as  Mars.  They  are 
all  visible  in  a  good  opera-glass.  Their  orbits  are  nearly  in  the 
plane  of  the  planet's  equator. 

246.  Eclipses.  —  Jupiter  casts  so  long  and  large  a  shadow  away 
from  the  sun  that  all  the  satellites  except  the  fourth  suffer  eclipse 
once    during   every  revolution.      The    times   of  these   eclipses   are 
given  in  the  Nautical  Almanac,  and    they  can  be  easily  observed 
with  a  small  telescope.     For  obvious  reasons  the  satellite  does  not 
disappear  instantaneously,  but  fades  from  sight  gradually. 

247.  Occultations.  —  When  a  satellite,  as  seen  from  the  earth,  is 
behind  Jupiter,  it  has  suffered  occultation.     When  it  has  been  just 
disappearing  behind  the  limb  of  Jupiter,  or  reappearing,  many  at- 
tempts have  been  made  to  see  it  shining  through  Jupiter's  atmos- 
phere, with  the  hope  of  measuring  its  refractive  power.     But  most 
of  the  attempts  (even  with  the  Lick  telescope)  are  acknowledged 
failures. 


JUPITER. 


169 


OCCULTATIOM 


248.  Transits. — A   satellite,  when  passing   between    us   and   the 
planet,  appears  to  cross  its  face  and  is  in  transit.      The   limb    of 
Jupiter  is  darker  than  the  cen- 
tre of  its  disk :  on  this  account 

a  satellite  is  visible  at  the  be- 
ginning or  end  of  a  transit  as 
a  bright  spot  on  a  dark  back- 
ground. When  the  satellite  is 
projected  upon  some  portion 
of  the  disk  which  has  the  same 
brightness  and  color,  it  be- 
comes invisible.  One  occa- 
sionally appears  dark,  or  even 
black  at  some  time  during  tran- 
sit: this  may  be  because  the 
background,  on  which  it  is  pro- 
jected at  the  time,  is  unusually 
bright.  The  shadows  of  the 
satellites  also  make  transits :  at 
times,  a  satellite  and  its  shadow 
are  seen  journeying  across  Ju- 
piter's face  in  company. 

249.  Markings  and  Rotation.— 

Fig.  139.  —  PHENOMENA  OF  THE  SATELLITES. 
Many  observers  have  reported 

dark  markings  on  the  satellites.     Some  of  the  most  authoritative 
recent  work  is  that  of  Professors  Schaeberle  and  Campbell l  with 


I 


Fig.  140.  —  MARKINGS  SEEN  WITH  THE  LICK  TELESCOPE. 

the  Lick  telescope.      Their  drawings  of  the  markings  on  satellite 
III,  made  with  very  high  magnifying  powers,  lend  strong  support 

1  W.  W.  Campbell,  astronomer  at  the  Lick  Observatory. 


OF  THE 


UNIVERSITY 


I7O  DESCRIPTIVE    ASTRONOMY. 

to  the  theory  that  it  continually  keeps  the  same  face  toward  Jupiter. 
Their  observations  also  show  that  satellite  I  is  perceptibly  elongated, 
and  that  its  long  axis  points  toward  the  planet's  centre.  These 
satellites  therefore  resemble  the  moon  in  that  their  times  of  revolu- 
tion and  rotation  are  coincident.  Schaeberle  and  others  have  seen 
a  bright  polar  cap  ort  satellite  III.  Barnard  discovered  a  white  belt 
on  satellite  I,  which  causes  it  to  appear  double  when  crossing  a 
white  part  of  Jupiter. 

250.  Pickering's  Observations  of  the  Satellites.  —  In     addition    to 
studying  the  belts,  Prof.  W.  H.  Pickering  has  made  careful  observa- 
tions of  the  satellites.     He  found  the  first  satellite  to  be  at  times 
very  plainly  elliptical,  the  major  axis  exceeding  the  minor  by  ten 
per  cent.     For  the  time  of  rotation  he  has  deduced  a  value  of  13  h. 
3  m.,  from  a  series  of  observations. 

The  rotation  time  of  the  second  satellite  was  much  more  difficult 
to  obtain,  but  a  value  of  over  41  hours  was  settled  upon. 

The  observations  of  the  third  and  fourth  satellites  favor  the 
theory  that  their  times  of  rotation  and  revolution  are  coincident. 

Prof.  Pickering's  results  are  in  only  partial  agreement  with  those 
of  other  observers :  the  matter  needs  much  further  study. 

251.  The  Fifth  Satellite.  —  On  Sept.  9,  1892,  Dr.  E.  E.  Barnard 
discovered  a  fifth  satellite,  with  the  Lick  telescope.     It  is  a  tiny 

point  of  light,  which  can  be  observed  with  the 
most   powerful    telescopes    only.      Its    distance 
from  the  centre  of  Jupiter  is  112,000  miles  and 
its  time  of  revolution  n  h.  57  m.  22. 56  sec.     Its 
Fig.  141.— JUPITER  AND    diameter  is  estimated  as   100  miles.     If  it  were 
THE  ORBIT  OF  THE     a   few  thousand  miles   nearer  to  the    planet,   it 

FIFTH  SATELLITE.  <  1  1111  •          •  1.1 

would  probably  be  torn  in  pieces  by  the  attrac- 
tion of  the  latter,  which  would  be  much  more  powerful  upon  the  side 
of  the  satellite  turned  toward  it  at  any  time  than  on  the  opposite  side. 

252.  Velocity  of  Light.  —The  fact  that  light  does  not  travel  from 
one   point  to   another   instantaneously  was   discovered  in   1675   by 
Roemer,  a  Danish   astronomer.     The  discovery  was  made  by  the 
discussion  of  observations  of  Jupiter's  satellites.     The  eclipses  recur 
at  nearly  equal  intervals.     By  noting  the  times  of  the  eclipses  of 
satellite    I,   for  example,   during   the    period    of  one   revolution   of 
Jupiter  about  the  sun,  one  can  calculate  with  great  accuracy  the 


JUPITER,    SATURN.  I  71 

average  interval  of  time  between  two  successive  eclipses :  the 
satellite  suffers  some  2,500  eclipses  during  that  period.  When 
Jupiter  is  in  opposition,  let  the  time  of  an  eclipse  be  noted :  by 
means  of  the  known  interval  between  two  successive  eclipses,  com- 
pute the  day,  hour,  and  minute  when  an  eclipse  will  occur  three 
months  after  opposition,  at  which  time  the  earth  will  be  farther 
from  Jupiter.  The  eclipse  will  happen  later  than  the  predicted  time. 
Predict  another  eclipse  near  the  time  of  the  planet's  conjunction  with 
the  sun,  when  the  earth  is  186,000000  miles  farther  from  Jupiter  than 
at  opposition.  The  eclipse  will  again  be  behindhand,  and  by  a  larger 
amount  than  before.  Roemer  sagaciously  guessed  that  the  eclipse 
which  took  place  near  the  time  of  conjunction  really  happened  on 
time,  but  that  the  light  which  brought  the  message  to  the  observer 
took  time  to  cross  the  extra  distance  of  186,000000  miles.  In  this  he 
was  right :  the  time  required  for  light  to  cross  the  earth's  orbit  is  close 
to  1,000  seconds. 

SATURN,    b. 

253.  Distance  and  Diameter.  —  The  distance  of  this  most  enchant- 
ing of  planets  from  the  sun  is  886,000000  miles.     Its  mean  diameter 
is  74,000  miles. 

254.  Revolution  and  Rotation The    sidereal    period    is    29^ 

years.      The  rotation  time   is  hard  to  determine  because  so  few 
small   well   defined   spots   have   ever   been    seen   on    its   surface. 
Prof.  Asaph  Hall  has  derived  a  value  of  loh.  14  m.   23. 8  sec.  from 
his  observations  of  a  white    spot  which  appeared  on    the  ball    in 
December,    1876,   and  was  visible  for  a  month. 

255.  Appearance  to  the  Naked   Eye.  —  Because  of  its  greater  dis- 
tance, Saturn  is  much  fainter  than  Jupiter.      It  alone  of  the  planets 
has  a  decided  yellowish  tint;  it  is  generally  as  bright  as  a  first 
magnitude  star,  and  may  be  distinguished  from  a  star  by  the  fact 
that  it  does  not  twinkle.     All  planets  have  this  peculiarity  except 
when  near  the  horizon.      A  person  who  is  acquainted  with  the  con- 
stellations may  find  it  easily  by  looking  up  its  right  ascension  and 
declination  in  the  Nautical  Almanac,  and  locating  it  on  a  star  map. 

256.  Telescopic  Appearance.  —  The  first   view  of  Saturn  with  a 
large  telescope  usually  calls  forth  an  exclamation  of  wonder  and 
delight.      For  the  globe  is  seen  to  be  surrounded  by  a  marvellous 


172  DESCRIPTIVE    ASTRONOMY. 

ring  system,  which  is  unique  in  the  solar  system,  and,  so  far  as 
we  know,  in  the  entire  universe.  A  goodly  retinue  of  satellites  is 
also  seen  attending  this  majestic  orb. 

The  ball  is  encircled  by  rather  bright  belts  near  the  equator, 
and  by  fainter  ones  at  higher  latitudes.  These  belts  are  not  sub- 
ject to  much  change  of  appearance,  except  that  due  to  imperfect 
seeing  caused  by  the  fluctuations  of  our  own  atmosphere.  Serenity 
is  natural  for  the  oldest  of  the  gods. 


Fig.  142.  — SATURN,  AS  SEEN  WITH  THE  LICK  TELESCOPE:  DRAWN  BY  KEELER. 

257.  Discovery  of  the  Kings.  —  In  1610  Galileo  discovered  that 
the  planet  appeared  triform.  This  was  due  to  the  imperfection  of 
his  telescope. 

He  said  that  Saturn  had  two  servants  who  aided  him  on  his  way. 
Great  was  his  perplexity  and  chagrin  to  find  that  the  attendants 
disappeared  after  a  year  or  two.  In  a  letter  to  a  friend  he  said :  — 

"What  is  to  be  said  concerning  so  strange  a  metamorphosis?  Are  the 
two  lesser  stars  consumed  after  the  manner  of  solar  spots?  Have  they 
vanished,  or  suddenly  fled?  Has  Saturn,  perhaps,  devoured  his  own 
children?  Or  were  the  appearances  indeed  illusion  and  fraud,  with 
which  the  glasses  have  so  long  deceived  me,  as  well  as  many  others  to 
whom  I  have  shown  them?  .  .  .  The  shortness  of  the  time,  the  unex- 
pected nature  of  the  event,  the  weakness  of  my  understanding,  have 
greatly  confounded  me." 


SATURN.  173. 

Nevertheless,  in  the  latter  part  of  the  letter  he  ventures  to  predict 
that  the  lost  bodies  will  reappear ;  and  he  himself,  as  we  learn  from 
a  later  letter,  saw  them  again  as  "  ears,"  one  on  each  side  of  the 
central  ball.  Forty  odd  years  later,  Huyghens,  a  Dutch  astronomer,, 
advanced  the  theory  that  Saturn  had  a  ring,  announcing  it  in  the 
form  "  aaaaaaa  ccccc  d  eeeee  g  h  iiiiiii  1111  mm  nnnnnnnnn  oooo 
pp  q  rr  s  ttttt  uuuuu."  These  letters,  properly  arranged,  form 
the  Latin  sentence,  Annulo  cingitur,  tenui,  piano,  nusquam  cohae- 
rente,  ad  eclipticam  inclinato.  The  translation  is :  "  It  is  encircled 
by  a  thin  flat  ring,  nowhere  touching,  inclined  to  the  ecliptic." 

258.  Divisions  and  Dimensions  of  the  Ring  System.  —  The   ring 
announced  by  Huyghens  has  been  found  with  powerful  telescopes, 
to  be    composed  of  three,  two  of  which  are  bright  and  the  third 
dark.     They  are  shown  clearly  in  Fig.  142.     The  division  between 
the  outer  ring  and  the  middle  one  has  been  named  Cassini's  di- 
vision, after  the  Italian  astronomer  who  first  noticed  it,  twenty  years, 
after  Huyghens's  announcement;  it  is  about  2,200  miles  in  width. 
There  is  a  finer  division  in  the  outer  ring,  known  as  the  Encke 
division.       Many   others    have   been    suspected.       One   has   been 
found  by  Keeler 1  at  the  Lick  Observatory. 

The  inner  ring  is  much  darker  and  fainter  than  the  others ;  it  is 
known  as  the  dark  or  dusky  ring.  The  extreme  diameter  of  the 
ring  system  is  173,000  miles,  and  its  breadth  is  about  equal  to  the 
semidiameter  of  the  ball.  The  rings  are  not  of  uniform  thickness,, 
as  is  shown  when  the  edge  of  the  system  is  turned  toward  us.  The 
average  thickness  is  not  far  from  one  hundred  miles. 

259.  Disappearance  of  the  Rings.  — This  phenomenon,  which  was 
so  sore  a  trial  to  Galileo,  is  explained  by    Fig.  143.     The  plane 
of  the  rings  coincides  with  that  of  the  planet's  equator,  but  is  in- 
clined 27°  to  the  plane  of   the  planet's  orbit.      Saturn  keeps  the 
successive  positions  of  its  rings  parallel  to  each  other  in  its  journey 
around  the  sun.     As  the  plane  of  the  earth's  equator  passes  through 
the  sun  in  March  and  September,  so  the  plane  of  Saturn's  rings- 
passes  through  the  sun  twice  in  one  of  its  revolutions.      Since  the 
earth,   as  seen  from  Saturn,   is  close  to  the  sun,  the  plane  of  the 
rings  will  pass  through  the  earth  within  a  few  weeks  of  the  same 
time.     At  such  a  time,  the  rings  disappear  because  they  are  thin, 

1  James  E.  Keeler,  Director  of  the  Allegheny  Observatory. 


174  DESCRIPTIVE    ASTRONOMY. 

and  edgewise  to  us.  The  disappearances  happen  at  intervals  of 
fifteen  years:  at  times  midway  between  them  the  rings  are  seen 
most  favorably. 


Fig.  143.  —  DIFFERENT  POSITIONS  OF  THE  RINGS. 

260.  The  Dark  Ring.  --  This  ring  is  sometimes  called  the  crape 
or  gauze  ring,  because  it  is  semi-transparent.     Dr.  Barnard,  at  the 
Lick  telescope,  on  Nov.  I,  1889,  observed  an  eclipse  of  one  of  the 
satellites.      After  emerging  from  the  shadow  of  the  ball  it  recovered 
its   normal  brightness,  and   soon  plunged   into  the  shadow  of  the 
dusky  ring;  it  then  became  fainter  and  fainter,  but  did  not  disap- 
pear until  it  had  passed  through  the  shadow  of  the  crape  ring  and 
into  the  shadow  of  the  inner  one  of  the  two  bright  rings ;  then  it  dis- 
appeared entirely.     This  shows  that  sunlight  sifts  through  the  dark 
ring,  and  that  the  transparency   of  the  latter  decreases  regularly 
from  its  inner  edge  to  its  outer,  where  it  joins  the  inner  bright  ring. 

261.  Structure  of  the  Rings.  —  The  researches  of  mathematicians 
have  demonstrated  that  neither  ring  can  be  an  unbroken  mass,  either 
solid  or  liquid.      In  either  case  the  ring  would  have  been  destroyed 
long   ago   by   the   attraction    of   the   ball.       The   hypothesis    now 
adopted  is  that  it  is  composed  of  myriads  of  minute  bodies,  a  con- 
geries of  closely  packed  moons,  each  of  which  has  an  orbit  of  its 
own.      In  the  dark  ring  the  bodies  are  less  closely  packed  together 
than  in  the  bright  ones.     At  the  outer  edge  of  the  dark  ring  they 
are  thought  to  be  more  densely  crowded  than  at  its  inner  edge. 


SATURN.  175 

This  hypothesis  concerning  the  structure  of  the  rings  has  been 
confirmed  by  observation.  The  separate  particles  are  much  too 
small  to  be  seen  separately,  but  their  existence  did  not  escape  the 
spectroscope  in  the  hands  of  Keeler  in  April,  1895.  If  the  bright 
ring  were  solid,  its  outer  edge  would  travel  faster  than  the  inner, 
just  as  a  point  on  a  tooth  of  a  circular  saw  moves  more  swiftly  than 
one  nearer  the  centre.  On  the  other  hand,  if  the  ring  is  made  up 
of  moonlets,  those  near  its  outer  edge  must  move  with  less  velocity 
than  those  near  the  inner.  Professor  Keeler 's  beautiful  photo- 
graphs of  the  spectrum  of  the  bright  ring  showed  that  the  outer  and 
inner  edges  had  respectively  velocities  of  10. 1  and  12. 4  miles  per 
second.  These  values  agree  well  with  theoretical  ones  computed 
according  to  Kepler's  Laws. 

262.  Stability  of  the  System.  —  The  Cassini  division  is  supposed 
to  be  due  to  the  attraction  of  the  largest  satellite,  which  has  changed 
the  orbits  of  the  bodies  which  once  occupied 

the  division.     The  outer  divisions  were  presum- 
ably caused  in  the  same  manner. 

These  minute  bodies  must  be  continually 
colliding  with  each  other,  so  that  some  of  them 
lose  velocity,  and  are  drawn  into  smaller  orbits  Fis-  J44-  —  OLD  DRAW. 

,         ,  .  c     ,       ,     ,.        rru  ING  OF  SATURN. 

by  the  attraction  of  the  ball.  The  appearance 
of  the  dark  ring  suggests  that  the  ring  system  is  being  thus 
disintegrated.  A  comparison  of  the  old  drawing  shown  in  Fig.  144 
with  Fig.  142  indicates  that  the  space  between  the  ball  and  the 
inner  edge  of  the  rings  is  now  smaller  than  formerly.  All  these 
considerations  have  led  to  the  hypothesis  that  Saturn  is  indeed 
u devouring  his  children." 

However,  no  evidence  of  such  a  change  is  given  by  accurate 
measures  of  the  dimensions  of  the  ring  made  during  the  past  one 
hundred  years. 

263.  The  Satellites.  —  These  are  eight  in  number.       Japetus,  the 
outermost,  is  at  a  distance  of  2,212000  miles,  and  occupies  seventy- 
nine  days  in  a  revolution.     Mimas,  the  innermost,  is  only  30,000 
miles  beyond  the  outer  edge  of  the  ring  system,  and  completes  its 
circuit  in  less  than  a  day.     Titan,  the  largest,  is  nearly  as  big  as 
Mercury.     All  move  in  the  plane  of  the  rings  excepting  Japetus, 
the  orbit  of  which  is  inclined   10°  to  it.     Japetus  suffers  remark- 


176  DESCRIPTIVE    ASTRONOMY. 

able  and  regular  variations  in  brightness,  which  are  explained  by 
assuming  that  one  hemisphere  of  it  is  much  brighter  than  the 
other,  and  that  it  always  presents  the  same  face  to  the  planet,  as 
our  moon  does  to  the  earth. 

264.  Physical  Condition  of  the  Planet.  —  The   mean   density   of 
the  ball  is  less  than  that  of  water,  or  even  alcohol,  closely  agreeing 
with  that  of  ether.     The  cloud  shell  surrounding  the  kernel  of  the 
planet  is  so  deep  that  it  hides  beneath  its  placid  exterior  nearly  all 
the   commotions   which   are   taking   place.     The    central    nucleus 
seems  to  possess  heat  sufficient  to  maintain  this  cloud  mantle,  but. 
not  sufficient  to  give  rise  to  such  activity  as  Jupiter  manifests. 
The  spectrum  of  the  ball  is  that  of  the  sunlight  reflected  from  its 
surface,   with   the    addition    of    some  dark    bands  caused   by   the 
absorption    of   the    sunlight    by  an   unknown    constituent    of   the 
planet's   atmosphere.      The   spectrum    of   the    rings   contains   no- 
absorption  bands. 

URANUS,    &  OR  #. 

265.  Discovery.1  —  William    Herschel     discovered    Uranus    in 
March,  1781.      He  was  an  organist  at  Bath,   England, —  a  man  of 
no   mean    musical    attainments.      In    studying    the    mathematical 
theory   of   music,   he   had   occasion  to   enlarge  his  knowledge    of 
mathematics ;   from  this  he  was  led  to  optics,  and  became  exceed- 
ingly   interested    in    telescopes   and   astronomy.      He  resolved  to 
make  a  reflecting  telescope :  supporting  himself  by  his  profession, 
he   devoted   his    leisure   to    grinding   and    polishing   specula   and 
lenses.      Rushing  home  from  a  concert,  he  would  plunge  at  once 
into  work  on  his  mirrors,  without  even  stopping  to  take  off   his 
lace  ruffles.      Mirror  after  mirror  was  constructed,  put  to  use,  and 
laid  aside  or  sold,  each  giving  place  to  a  new  one,  more  perfect,  or 
of  larger  size.     When  engaged  in  putting  the  finishing  touches  on 
one  of  his  great  mirrors,  he  often  sat  at  his  work  for  hours,  food 
being  put  into  his  mouth  by  his  devoted  sister  Caroline,  who  sat 
by   his   side,  and   beguiled   the  time  by  reading   "The   Arabian 
Nights." 

1  This  article  is  chiefly  a  condensation  of  material  found  in  Ball's  "Story  of  the: 
Heavens." 


URANUS.  177 

Such  unremitting  enthusiasm  and  genius  must  find  their  reward. 
After  half  a  dozen  years  of  this  assiduous  toil,  he  succeeded  in 
constructing  a  seven-inch  reflector  of  exquisite  optical  perfection. 
He  resolved  to  examine  all  the  stars  above  a  certain  order  of 
brightness.  Now  a  fixed  star  is  the  merest  point  of  light;  the 
more  perfect  the  telescope,  the  smaller  is  the  image  of  the  star. 

Star  after  star  passed  in  review  before  him.  Finally,  on  the 
night  of  March  13,  1781,  he  perceived  an  object  which  looked  like 


Fig.  145.  —  SIR  WILLIAM  HERSCHEL. 

a  star,  except  that  its  disk  was  a  trifle  larger  than  that  of  a  star  of 
the  same  brightness.  Many  a  time  had  this  object  been  observed 
by  other  astronomers,  but  they  had  noticed  no  peculiarity  in  its 
appearance.  Herschel  soon  found  that  it  was  in  slow  motion;  he 
reasoned  that  it  must  be  nearer  than  the  fixed  stars,  and  not 
dreaming  that  he  had  discovered  a  new  major  planet,  the  others 
having  been  known  since  the  dawn  of  astronomical  science,  an- 
nounced that  he  had  found  a  comet.  Astronomers  at  once  set  to 


12 


178  DESCRIPTIVE    ASTRONOMY. 

work  observing  it.  Computations  of  its  orbit  followed.  Within 
a  year  the  mathematicians  had  demonstrated  that  the  orbit  was 
nearly  a  circle,  twice  as  large  as  the  path  of  Saturn.  The  object 
was  therefore  a  planet.  Herschel  proposed  the  name  Georgium 
Sidus  in  honor  of  his  sovereign;  Laplace  suggested  the  designa- 
tion Herschel.  The  name  finally  adopted  was  proposed  by  Bode. 
This  notable  extension  of  the  confines  of  the  solar  system  was 
hailed  with  the  greatest  enthusiasm.  King  George  knighted 
Herschel,  and  gave  him  ,£200  a  year.  The  further  career  of  Her- 
schel, who  finally  constructed  a  reflector  four  feet  in  aperture  and 
forty  feet  in  focal  length,  stamps  him  as  foremost  among  astronom- 
ical observers. 

266.  Distance  and  Diameter.  —  The  distance  of  Uranus  from  the 
sun  is   1,782,000000    miles.      Its  diameter    is    32,000   miles,   four 
times  that  of  the  earth. 

267.  Revolution  and  Rotation.  — •  The  sidereal   period  is  eighty- 
four  years.      The   time   of  rotation  is  unknown,  because  no  suffi- 
ciently definite  markings  have  ever  been  seen  on  its  surface. 

268.  Appearance.  —  To  the  naked  eye  it  appears  as  a  small  star 
just  on  the  limit  of  visibility.      It  may  be  found  by  the  use  of  the 
Nautical  Almanac,  which  gives  its  right  ascension  and  declination 
throughout  the  year. 

In  a  large  telescope  it  exhibits  a  greenish  disk  occasionally 
marked  by  faint  belts.  Granting,  as  mathematical  theory  de- 
mands, that  the  planes  of  the  orbits  of  the  satellites  nearly  coin- 
cide with  that  of  the  planet's  equator,  the  belts  are  not  parallel  to 
the  equator,  an  unexplained  anomaly. 

269.  The  Satellites.  —  Uranus  is  attended  by  four  of  these  bodies, 
no  one  of  which  can  be  seen  by  an  ordinary  eye  with  a  telescope 
less  than  eight  inches  in  aperture.      The  diameter  of  the  largest  is 
probably  five  hundred  miles.      Their  orbits  lie  in  one  plane,  which, 
strange  to  say,  is  nearly  perpendicular  to  the  ecliptic.      They  also 
revolve  backwards,  that  is  from  east  to  west,  in  their  orbits,  unlike 
any  other  satellites  before  considered. 

270.  Physical  Condition.  —  Of  this  little  is  known.     The  spec- 
trum of  the  planet  exhibits  some  conspicuous  bands,  thought  to  be 
due  to  the  absorption  of  a  dense  atmosphere.      Sunlight  at  Uranus 
being  only  3- J-g-  as  intense  as  at  the  earth,  the  processes  of  cloud 


NEPTUNE.  179 

formation  must  be  dependent  chiefly  on  internal  heat.      Its  mean 
density  is  less  than  that  of  bituminous  coal. 

NEPTUNE,  ^. 

271.  Discovery.  —  The  discovery  of  Neptune  is  esteemed  the 
most  notable  triumph  of  mathematical  astronomy.  It  was  no  mere 
accident,  nor  was  it  brought  about  simply  by  a  diligent  search  with 
the  telescope.  Forty  years  after  the  discovery  of  Uranus,  Bou- 
vard,  a  French  astronomer,  published  tables  of  its  motion,  by 
means  of  which  its  place  could  be  predicted  for  the  future.  But 
the  planet  refused  to  follow  the  path  marked  out  for  it;  farther 
and  farther  it  departed  from  the  appointed  course.  In  twenty  years 
the  discrepancy  between  theory  and  observation  had  become  intol- 
erable. To  be  sure,  the  difference  could  not  yet  be  perceived  by 
the  naked  eye,  but  the  unfailing  accuracy  of  the  observations 
loudly  proclaimed  that  there  was  some  fault  in  the  theory  of  the 
planet's  motion.  Was  the  law  of  gravitation  partially  inoperative 
at  this  enormous  distance  from  the  sun  ?  Had  a  flaw  been  found 
at  last  in  the  marvellous  researches  of  Newton  ?  By  no  means. 
From  many  quarters  came  the  suggestion  that  some  unknown  body 
was  displacing  Uranus  by  its  powerful  attraction.  But  could  the 
position  of  the  troublesome  stranger  be  pointed  out? 

John  Couch  Adams,  a  tutor  in  the  University  of  Cambridge, 
England,  grappled  with  the  problem.  In  October,  1845,  ne  com- 
municated to  the  Astronomer  Royal  of  England  the  elements 
of  the  orbit  of  the  suspected  planet,  together  with  a  prediction  of 
its  place  in  the  sky.  But  the  Astronomer  Royal l  did  not  regard 
these  investigations  of  a  young  and  comparatively  unknown  man 
as  entitled  to  much  confidence.  He  however  called  the  attention 
of  a  few  of  his  friends  to  them,  and  wrote  Adams  asking  some 
further  information  :  no  reply  reached  him.  He  therefore  pigeon- 
holed the  manuscript.  One  of  the  friends  wrote  to  Lassell,  who 
possessed  a  fine  two-foot  reflector  which  was  mounted  near  Liver- 
pool, begging  him  to  search  for  the  planet.  But  Lassell  was 
suffering  from  a  sprained  ankle,  and  when  he  recovered,  the  letter 
was  nowhere  to  be  found,  and  the  telescopic  search  was  not  made. 

1  Sir  George  Biddell  Airy. 


i8o 


DESCRIPTIVE    ASTRONOMY. 


Meanwhile  Leverrier,  a  brilliant  French  astronomer,  likewise  a 
young  man,  had  employed  his  powers  upon  the  same  problem. 
On  June  i,  1846,  he  sent  a  communication  to  the  French  Academy 
of  Sciences  giving  the  direction  in  which  the  planet  was  to  be 
found. 


Fig.  146.  —  JOHN  COUCH  ADAMS. 

The  English  astronomers,  finding  that  Leverrier's  results  agreed 
with  those  of  Adams,  awoke  from  their  lethargy,   and  began  to 


NEPTUNE.  l8l 

bestir  themselves.  Professor  Challis,  the  astronomer  of  the 
University  of  Cambridge,  commenced  a  search.  Doubting  the 
accuracy  of  the  predictions,  he  began  to  map  a  large  area  of  the  sky, 
hoping  by  comparison  of  maps  of  the  same  region  made  on  different 
nights  to  detect  the  planet  by  its  change  of  position  if  it  were 
really  there. 

Sir  John  Herschel  (son  of  Sir  William),  in  a  public  address,  said 
concerning  the  unknown  body:  "We  see  it  as  Columbus  saw 
America  from  the  coast  of  Spain.  Its  movements  have  been  felt, 
trembling  along  the  far-reaching  line  of  our  analysis,  with  a  cer- 
tainty hardly  inferior  to  that  of  ocular  demonstration." 

Three  times  Challis  observed  the  planet,  but  did  not  look  sharply 
enough  to  notice  its  disk,  which  was  larger  than  that  of  the  stars. 
While  he  was  laboriously  heaping  up  observations  and  neglect- 
ing to  compare  them,  the  prize  of  discovery  slipped  from  his  grasp. 
Leverrier  had  written  to  Galle,  of  Berlin,  where  some  excellent 
star  charts  were  being  made,  asking  him  to  direct  his  telescope  to 
a  certain  point  on  the  ecliptic,  and  saying  that  he  would  find 
within  a  degree  of  that  point  a  new  planet,  as  bright  as  a  star  of 
the  ninth  magnitude  (§  i)  and  having  a  perceptible  disk.  Galle 
did  as  he  was  bidden,  and  found  the  planet  within  half  an  hour,  on 
Sept.  23,  1846.  Success  is  to  the  confident. 

272.  Distance   and  Diameter.  —  The  mean    distance  of  Neptune 
from    the    sun    is    2,792,000000   miles.      It    is    therefore  a  billion 
miles  farther  than  Uranus,   and  thirty  times  as  far  as  the  earth. 
The  diameter  is  35,000  miles. 

273.  Revolution  and  Rotation.  — The  sidereal  period  is  nearly  165 
years.     The  time  of  rotation  is  unknown,  because  no  well  defined 
spots  have  ever  been  seen  on  the  surface. 

274.  Appearance.  —  Neptune  is  too   faint  to  be    visible   to  the 
naked  eye.      A  good  opera-glass  will  show  it.      It  may  be  found  by 
using  the  Nautical  Almanac  and  a  star  map,  as  formerly  explained 
(§  255).      In  a  large  telescope  its  greenish  disk  is  readily  perceived, 
but  no  marks  have  been  seen  upon  it. 

275.  Satellite. — There  is  one  satellite,  a  very  faint  object,  sup- 
posed to  be  of  the  size  of  our  moon.     The  plane  of  its  orbit  is  in- 
clined 35°  to  the  ecliptic,  and  the  satellite,  like  those  of  Uranus, 
moves  backwards  from  east  to  west. 


1 82  DESCRIPTIVE    ASTRONOMY. 

276.  Physical  Condition.  —  The  spectrum   is    similar   to  that  of 
Uranus,    showing  faintly   the    same    absorption    bands,   which  are 
presumably  due  to  a  dense  atmosphere.     The  sunlight  is  only  9^ 
as  intense  as  at  the  earth ;    perhaps    no  cheering  ray  of   sunlight 
penetrates  the  clouds  in  which  the  planet  is    entirely  enveloped. 
The  density  of  Neptune  is  a  little  less  than  that  of  Uranus.     The 
two  planets  are  almost  identical  in  size  and  general  make  up. 

277.  Planets  beyond  Neptune.  —  Such  planets  have  been  suspected 
on  various   insufficient  grounds ;  they  have  been  hunted   for  with 
large  telescopes,  both  visually  and  by  means  of  photography,  which 
brings  to  light  stars  too  faint  to  be  seen  with  the  most  powerful 
telescopes.       No    success    has    yet    attended   these   efforts.     The 
24-inch  Bruce  photographic  telescope,1  if  used  for  long  exposure 
photographs  in  the  vicinity  of  the  ecliptic,  would  reveal  hosts  of 
new  asteroids,  and  might  bring  to  notice  ultra-Neptunian  planets. 

EXERCISES. 

278.  i.    Why  is  Jupiter's  disk  elliptical? 

2.  The  volumes  of  spheres  are  to  each  other  as  the  cubes  of  their 
radii  or  diameters.     Verify  the  statement  that  Jupiter  is  i,  300  times 
as  large  as  the  earth. 

3.  What  reasons  are  there  for  thinking  that  Jupiter  has  no  solid 
crust  ? 

4.  Though  Jupiter  is  i,  300  times  as  large  as  the  earth,  its  density 
is  only  o.  24  as  great.      Its  mass  is  therefore  how  many  times  that 
of  the  earth  ? 

5.  Ought  Jupiter  to  appear  gibbous,  when  at  quadrature,  like 
Mars  ? 

6.  Why  do  Jupiter's  belts,    if  they  are  due  to  the  absence  of 
clouds,  look  darker  than  the  cloudy  portions? 

7.  If  the  interior  of  Jupiter  were  so  hot  as  to  shine  through  his 
atmosphere,  wouJd  the  spectrum  be  continuous,  or  crossed  by  dark 
lines  ? 

8.  Can  one  of  Jupiter's  satellites  be  in  occupation  and  also  in 
eclipse  at  the  same  time? 

1  By  far  the  most  powerful  telescopic  camera  in  existence,  —  the  property  of  Harvard 
College  Observatory. 


JUPITER,  SATURN,  URANUS,  NEPTUNE.          183 

9.  (a)  If  one  of  Jupiter's  satellites  were  in  transit,   and  were 
almost  exactly  between  us  and  its  own  shadow,  would  Jupiter  be 
near  opposition  or  near  quadrature  ? 

(b}  Might  it  be  near  conjunction  ? 

10.  (a)  Why  is  an  eclipse  of  one  of  Jupiter's  satellites  not  instan- 
taneous ? 

(b)  Is  an  occupation  instantaneous? 

11.  If  Jupiter's  atmosphere  were  sufficiently  transparent  to  let 
the  light  of  a  satellite  through  when  it  was  disappearing  in  occulta- 
tion,  would  the  time  of  disappearance  be  delayed  ? 

12.  Would  the  refractive  power  of  Jupiter's  atmosphere  delay 
the   time   at   which   a   satellite   entered   upon   a   transit   over  his 
disk? 

13.  Our  moon,  when  eclipsed,  is  usually  visible.      Why  are  not 
eclipsed  satellites  of  Jupiter  similarly  visible? 

14.  Suppose  that  the  shadow  of  one  of  Jupiter's  satellites,  when 
moving  across  its  disk,  fell  upon  some  portion  that  was  decidedly 
brighter  than  the  average,  would  the  shadow  look  darker  in  conse- 
quence, or  lighter? 

15.  A  person  on  Jupiter,  in  the  shadow  of  one  of  its  satellites, 
would  see  an  eclipse  of  what  ? 

16.  If  satellite  I  was  once,  or  is  now,  a  fluid  mass,  why  is  it 
elongated  ? 

17.  If  satellite  I  was  once  fluid,  and  rotated  more  swiftly  than 
now,  what  force  has  checked  its  velocity  of  rotation  ? 

1 8.  If  satellites  III  and  IV  were  fluid,  being  of  the  same  size 
and  composition,  why  should  III  be  more  elongated  than  IV? 

19.  What  is  the  velocity  of  light,  in  miles  per  second,  on  the 
assumption  that  light  takes  just  1,000  seconds  to  cross  the  earth's 
orbit? 

20.  If  Jupiter's  fifth  satellite  has  the  same  albedo  as  satellite 
IV,  that  is,  reflects  sunlight  just  as  well,  state  two  reasons  why  it 
is  difficult  to  see. 

21.  The  volume  of  Saturn  is  how  many  times  that  of  the  earth, 
if  its  diameter  is  nine  times  as  great? 

22.  The  earth's  diameter  being  taken  as  unity,   the  diameters 
of  the  other  planets  are  roughly  as  follows :  Mercury  -J,  Venus  I, 
Mars  J,  Jupiter  11,  Saturn  9,  Uranus  4,  Neptune  4^.      Show  that 


184  DESCRIPTIVE    ASTRONOMY. 

the  volume  of  Jupiter  is  greater  than  that  of  all  the  other  major 
planets  together. 

23.  Assuming  the  approximate  data  in  the  preceding  exercise, 
find  whether  the  surface  of  Jupiter  is  as  great  as  the  combined 
surfaces  of  the  other  major  planets. 

24.  Could  we  ever  see  an  occultation  of  Mars  by  Jupiter? 

25.  Why  did  Galileo's  two  attendants  of  Saturn  disappear? 

26.  What  appearance  in  Fig.   142  shows  that  the  dark  ring  of 
Saturn  is  transparent  ? 

27.  Is  the  shadow  of  the  ball  of  Saturn,  as  cast  upon  the  rings, 
visible  in  Fig.   142? 

28.  Is  the  shadow  of  the  bright  rings  of  Saturn,  cast  upon  the 
ball,  visible  in  Fig.   142? 

29.  Does  the  plane  of  Saturn's  rings,  when  extended  indefinitely, 
ever  pass  between  the  earth  and  the  sun? 

30.  Does  the  sun  ever  illuminate  both  sides  of  Saturn's  rings  at 
the  same  time  ? 

31.  Is  one  side  of  Saturn's  rings  perpetually  unilluminated  by 
the  sun? 

32.  If  the  plane  of  the  rings  ever  passed  between  the  sun  and 
the  earth,  could  we  then  see  the  bright  side  of  the  rings  ? 

33.  Suppose  that  the  ball  of  Saturn  was  a  perfect    sphere    of 
uniform  density  throughout ;  also  that  the  ring  system  was  a  solid 
sheet   of   matter,    truly    circular,    uniform   in   both    thickness   and 
density,  and  concentric  with  the  ball ;  suppose  further  that  one  of 
the  satellites  attracting  the  ring  pulled  it  to  one  side,  so  that  it 
was  no  longer  concentric  with  the  ball,  would  the  attraction  of  the 
ball  pull  it  farther  until  it  struck  the  surface  of  the  ball  ? 

34.  Would  a  great  difference  in  brightness  between  the  outer 
edge  of  Saturn's  dusky  ring  and  the  inner  edge  of  the  bright  ring 
next  to  it  militate  against  the  theory  advanced  in  §  262,  that  Saturn 
is  "devouring  his  chijdren  "  ? 

35.  What  does  the  absence  of  absorption  bands  from  the  spec- 
trum of  the  rings  indicate  concerning  their  atmosphere? 

36.  Why  is   the    direction   east   to    west   called    backwards  in 

§  269? 

37.  How  can  the  statement  in  §  276,  that  sunlight  at  Neptune 
is  only  -g-J-g-  as   intense  as  at  the  earth,    be  figured  out   from  the 


JUPITER,    SATURN,    URANUS,    NEPTUNE.  185 

statement  in  §  272,  that  Neptune's  distance  is  thirty  times  that  of 
the  earth  ? 

38.  Ought  Neptune  to  look  very  gibbous  when  a*  quadrature? 

39.  When   Neptune   is  at   opposition,  show  that  the   light   by 
which  we  see  it  left  the  sun  8  h.  1 1  m.  40  sec.  before  it  reached  us. 
Assume  that  light  takes  five  hundred  seconds  to  come  from  the  sun 
to  the  earth,  and  that  Neptune's  distance  from  the  sun  is  thirty 
times  ours. 

40.  Sunlight  at  Neptune  would  be  how  many  times  as  intense 
as  our  moonlight  ?     (§  165.) 

41.  (a)  Does  Neptune  disturb  the  motion  of  Mercury  at  all? 
(&)  Does  Mercury  disturb  that  of  Neptune  ? 


l86  DESCRIPTIVE    ASTRONOMY. 


CHAPTER   XL 

COMETS   AND   METEORS. 

"  Stranger  of  heaven,  I  bid  thee  hail ! 
Shred  from  the  pall  of  glory  riven, 
That  flashest  in  celestial  gale, 
Broad  pennon  of  the  King  of  Heaven  ! " 

HOGG. 

279.  Comets  in  General.  — The  word  "comet  "  is  derived  from  a 
Greek  word,   which   means  the  long-haired  one;    the  designation 
evidently  came   from   the    resemblance  of   the  tail  to  dishevelled 
tresses.      These    bodies   are  very  different  in   behavior   from    the 
staid  and  trusty  planets.     They  usually  come  unheralded,   change 
their  form  and  brightness  from  night  to  night,   display  all  their 
antics  in  a  few  weeks  or  months,  and  are  off  again,  perchance  to 
whisk  about  some  other  world  in  like  gay  fashion. 

280.  Discovery.  —  In  the    early    ages   only    those    comets   were 
discovered  which  were  bright  enough   to    be   conspicuous  to   the 
naked    eye.      But    of    late   years  a  comet  does   not   often    become 
visible  to  the  naked  eye  before  one  of   the  comet-hunters l    has 
detected  it  with  his  telescope. 

These  observers  usually  employ  small  telescopes  equipped  with 
low  powers,  so  that  the  field  of  view  may  be  large.  Hour  after 
hour  they  scan  the  face  of  the  sky,  hunting  for  nebulous-looking 

1  The  following  extract  about  Messier,  a  comet-hunter  of  the  eighteenth  century,  is 
taken  from  Langley's  New  Astronomy ;  it  is  given  there  as  a  translation  from  Delambre's 
History  of  Astronomy :  "  He  has  passed  his  life  in  nosing  out  the  tracks  of  comets. 
He  is  a  very  worthy  man,  with  the  simplicity  of  a  baby.  Some  years  ago  he  lost  his 
wife,  and  his  attention  to  her  prevented  him  from  discovering  a  comet  he  was  on  the 
search  for,  and  which  Montaigne  of  Limoges  got  away  from  him.  He  was  in  despair. 
When  he  was  condoled  with  on  the  loss  'he  had  met,  he  replied,  with  his  head  full  of 
the  comet,  *  Oh  dear !  to  think  that  when  I  had  disco'vered  twelve,  this  Montaigne 
should  have  got  my  thirteenth ! '  And  his  eyes  filled  with  tears,  till,  remembering  what 
it  was  he  ought  to  be  weeping  for,  he  moaned,  '  Oh  my  poor  wife  ! '  but  went  on  crying 
for  his  comet." 


COMETS    AND    METEORS.  187 

objects.  Faint  comets  ordinarily  look  so  much  like  nebulae  that 
they  cannot  be  distinguished  from  them,  except  by  their  motion. 
A  comet-hunter,  finding  such  an  object,  looks  at  his  catalogue  of 
nebulae  to  see  if  it  is  given  there.  If  not,  it  may  be  a  comet, 
and  he  watches  it  until  he  has  found  out  whether  it  is  in  motion; 
if  in  motion,  he  announces  it  as  a  comet. 

Photography  has  now  scored  its  first  success  in  this  field. 
Dr.  Barnard  was  the  first  to  discover  a  comet  by  photography.1 
Special  photographic  lenses  are  employed  which  enable  the 
astronomer  to  photograph  on  one  plate  a  large  region  of  the 
sky.  One  drawback  to  this  method  is  that  the  exposure  times  are 
necessarily  long,  since  a  faint  object  does  not  impress  itself  on 
the  plate  quickly. 

281.  Number :  Designation.  —  During  the  past  three  thousand 
years  there  have  been  recorded  about  seven  hundred  of  these  bod- 
ies. Before  the  invention  of  the  telescope  the  rate  of  discovery  was 
slow,  because  only  a  few  comets  are  conspicuous  objects  to  the 
naked  eye.  At  present  about  half  a  dozen  are  found  annually, 
the  majority  of  them  being  merely  telescopic,  i.  e.  too  faint  to  be 
seen  without  a  telescope. 

There  may  be  thousands  of  comets  which  never  come  near 
enough  to  the  earth  to  be  discovered.  It  has  been  estimated  that 
millions  of  them  never  come  nearer  to  the  sun  than  Neptune  does. 
Kepler  thought  comets  to  be  as  numerous  in  the  heavens  as  fishes 
in  the  ocean. 

Comets  especially  noteworthy  receive  special  names.  The  great 
comet  of  1858  received  the  name  of  Donati's  comet,  Donati  being 
its  discoverer.  Encke's  comet  was  named  for  him,  because  he 
made  some  striking  researches  concerning  its  movements.  The 
wonderful  comet  found  by  Finlay,  at  the  Cape  of  Good  Hope,  in 
the  fall  of  1882,  was  so  majestic  that  no  man's  name  has  been 
attached  to  it.  It  is  known  as  "The  Great  Comet  of  1882." 

Other  designations  are  used  for  the  convenience  of  astronomers. 
Comet  a  1892  denotes  the  first  comet  discovered  in  that  year: 
Comet  f  would  be  the  sixth  comet.  Roman  numerals  are  also 

1  So  faint  was  the  photographic  impression  that,  when  another  keen-sighted  astrono- 
mer was  asked  to  find  the  comet  on  the  plate,  he  was  unable  to  do  so,  though  he 
succeeded  in  seeing  it  after  it  was  pointed  out  to  him. 


1 88  DESCRIPTIVE    ASTRONOMY. 

used  to  denote  the  order  of  arrival  at  perihelion.      Comet  1889  V. 
was  the  fifth  in  that  year  to  arrive  at  its  perihelion. 

282.  Brightness  and  Visibility.  —  Comets  vary  greatly  in  bright- 
ness,  some  being  so  faint  that  only  a  powerful  telescope  reveals 
them,  others  being  so  brilliant  that  they  can  be  seen  in  full  day- 
light though  close  to  the  sun.      The  brightness  continually  changes 
as  the  distances  of  the  comet  from  the  earth  and  sun  change.      But 
there  are  also  irregular  fluctuations  of  brightness. 

There  is  rarely  a  week  through  the  year  when  some  comet  is  not 
in  sight.  Some  of  them  are  seen  only  a  few  weeks  before  they 
become  too  faint  to  be  observed  longer.  But  the  large  telescopes 
scattered  throughout  the  world  now  enable  astronomers  to  follow 
comets  for  a  much  longer  time  than  before.  One  comet  was  seen 
with  the  Lick  telescope  over  two  years  after  its  discovery. 

283.  Parts  of  a  Comet.  —  Three  parts  of  a  comet  are  usually  men- 
tioned, —  the  coma,  or  head,  the  nucleus,  and  the  tail. 

The  coma  is  the  cloudlike  form,  which  is  the  distinguishing 
mark  of  a  comet.  Faint  comets  are  frequently  all  coma,  no  tail 
or  nucleus  being  visible. 

The  nucleus  is  a  starlike  or  planetary  point  in  the  coma,  the 
most  condensed  portion  of  the  comet.  It  is  likewise  the  most 
brilliant  part,  and  usually  contains  most  of  the  comet's  mass. 

The  tail  is  the  train  of  tenuous  matter  which,  streaming  from 
the  head,  is  the  chief  glory,  to  the  naked  eye,  of  a  large  comet. 

284.  Forms  of  Orbits.  —  If   a    right   triangle  be  revolved    about 
one  of  its  perpendicular  sides  as  an  axis,  it  will  generate  a  cone. 
The  base  of  the  cone  will  be  a  circle  (Fig.   147).      A  section  CD  of 
the  cone,   made  by  a  plane  parallel  to  the  base,  is  a  circle.      If  the 
cutting  plane  is  inclined  to  the  base  by  a  less  angle  than  VAB,  the 
section  EM  is  an  ellipse.     When  the  cutting  plane  FGH  is  parallel 
to  VA,  the  section  is  a  parabola.     A  plane  IKL,  which  is  more 
nearly  parallel  to  the  axis  VO  than  FGH  was,  cuts  an  hyperbola 
out  of    the  conical   surface.      The    circle   and    ellipse   are    closed 
curves,  but  the  parabola  and  hyperbola  are  not. 

The  three  curves  are  delineated  in  Fig.   148.     The  branches  of 
a  parabola  become  more  nearly  parallel  to  each  other  the   farther 
they  extend.      The  branches  of  an  hyperbola  continually  diverge. 
A  comet  which  travels  in  an  ellipse  will  return  to  our  vision  at 


COMETS    AND    METEORS. 


189 


regular  periods,  unless  it  undergoes  some  powerful  disturbance, 
which  alters  its  patn.  A  parabolic  or  hyperbolic  comet  never 
returns. 


L  H 

Fig.  147.  —  CONIC  SECTIONS. 


Fig.  148.  —  VARIETIES  OF  ORBITS. 


285.  Significance  of  these  Forms.  —  Suppose  that  a  small  body 
is  at  a  very  great  distance  from  the  sun,  and  both  bodies  are 
motionless.  The  body  will  begin  to  fall  toward  the  sun,  its  path 
being  a  straight  line  directed  toward  the  sun's  centre.  Another 
small  body,  likewise  at  a  distance  practically  infinite,  has  a  slight 
motion  of  its  own,  but  is  not  moving  directly  toward  the  sun; 
urged  on  by  the  sun's  imperious  attraction,  its  velocity  will  contin- 
ually increase;  however,  as  it  is  not  going  directly  toward  the  sun, 
it  will  not  strike  it,  but  as  it  goes  past  the  pull  of  the  sun  will 
cause  its  path  to  be  violently  curved ;  whirling  around  the  sun,  it 
will  return  toward  the  infinite  depths  of  space  from  which  it  came ; 
its  orbit  is  a  parabola.  A  body  which  has  originally  a  very  con- 
siderable velocity  of  its  own  will  come  down  to  the  sun  in  an 
hyperbolic  orbit,  and  then  retreat,  never  again  to  visit  us. 

A  body  moving  in  a  parabola  may  have  its  velocity  checked,  as 
it  approaches  the  sun,  by  the  attraction  of  some  planet :  its  orbit 
will  thus  be  changed  to  an  ellipse.  Were  the  movement  of  the 
body  accelerated  by  the  planet's  action,  the  orbit  would  become  an 
hyperbola. 


DESCRIPTIVE    ASTRONOMY. 

286.  Groups  of  Comets A  comparison  of  the  orbits  of  comets 

shows  that  there  are  certain  groups  of  them,  pursuing  nearly  the 
same  paths.     The  Great  Comet  of  1882  belonged  to  one  of  these 
groups,  the  orbits  of  those  of  1668,  1843,  1880,  and  1887  being  very 
similar  to  its  orbit.      Since  each  of  these  four  comets  approaches 
very   close  to  the   sun's   surface,   when  at   perihelion,   a   startling 
theory  was  promulgated  in  1882  that  these  were  one  and  the  same, 
the  periodic  time  being  continually  lessened  by  passing  through  the 
sun's  atmosphere,  and  that  the  comet  would  plunge  into  the  sun 
in  a  few  months.      It  is  needless  to  say  that  this  anticipation  was 
not  verified.     Tisserand l  has  recently  shown  that  comet  groups  may 
be  caused  by  the  disruption  of  the  nucleus  of  a  single  comet,  in 
consequence  of  the  heat  or  tidal  action  of  the  sun.     The  fragments 
would  thereafter  pursue  very  similar  orbits,   the    chief  difference 
being   in  the  periodic  times.     A  comet    moving  in  an  ellipse  is 
called  aperiodic  comet,  because  it  returns  at  regular  intervals. 

287.  Planetary  Families  of  Comets.  —  Fig.   149  shows  the  orbits  of 
a  number  of  recent  periodic  comets.      Inspection  shows  that  the 
aphelion  of  each  orbit  lies  near  the  orbit  of  Jupiter.      This  fact 
suggests  that  Jupiter  is  the  planet   which   retarded  these  comets 
as  they  were  sweeping  down  towards    the    sun    from    interstellar 
space,  and  transformed  their  orbits  into  ellipses. 

Jupiter,  having  a  much  more  powerful  attractive  force  than  any 
other  planet,  is  credited  with  a  larger  family  of  comets,  about  twenty 
individuals  in  all.  Neptune,  the  outpost  of  the  solar  system,  has 
succeeded  in  capturing  half  a  dozen. 

It  should  be  borne  in  mind  that  a  planet's  influence  does  not 
always  work  in  favor  of  a  comet's  capture.  The  planet  may  be 
so  placed  with  reference  to  the  comet  as  to  accelerate  its  motion. 

288.  Successive    Changes  in  Orbits :  Exact    Parabolas.  —  A  comet 
which  approaches  near  a  planet  and  suffers  a  change  of  orbit  may 
come  near  it  again  after  a  few  years  and  suffer  another   change. 
An    ellipse   may  be  changed  into    another  ellipse,    in   which  the 
comet    revolves  in  a  longer  or  shorter  period   than    formerly.       A 
number  of  such  instances  are  known.      Comet   1889  V.  (see  §292) 
was  in    1884   revolving    in   an    ellipse    having   a  period   of  about 
thirty  years.      In    1886    it    came    so    near  to  Jupiter  that   it   was 

1  Late  Director  of  the  Paris  Observatory. 


COMETS    AND    METEORS. 

for  a  short  time  almost  dominated  by  that  planet,  and  described 
an  hyperbola  about  it ;  when  it  finally  escaped  from  Jupiter,  and 
yielded  to  the  power  of  the  sun  again,  its  period  was  only  seven 
years.  In  1921  Jupiter  will  modify  its  orbit  seriously  again,  prob- 
ably enlarging  it  greatly.  The  ellipse  may  become  a  parabola  or 
even  an  hyperbola.  When  this  is  said,  it  must  be  remembered 


Fig.  149.  —  ORBITS  OF  SOME  COMETS  OF  JUPITER'S  FAMILY. 

that  an  orbit  is  called  a  parabola  when  its  form  approaches  that 
curve  so  nearly  that  our  observations  detect  no  appreciable  devia- 
tion from  it.  The  chances  are  that  no  orbit  is  an  exact  parabola, 
for  if  a  comet  were  moving  at  any  instant  in  a  parabolic  orbit, 
the  slightest  attraction  from  any  body  (it  must  suffer  many  such 
pulls)  would  change  the  orbit  into  an  ellipse  of  very  long  period, 
or  an  hyperbola.  The  orbits  of  most  comets  are  sensibly  parabolic. 
289.  Changes  of  Appearance. — When  a  telescopic  comet  is  first 
seen,  it  appears,  as  has  been  said,  like  a  filmy  cloud  on  the  bosom 


DESCRIPTIVE    ASTRONOMY. 

of  the  sky.  The  coma  usually  looks  more  condensed  toward  the 
centre.  As  it  approaches  the  sun,  it  grows  brighter,  and  the 
nucleus,  if  it  has  any,  comes  into  view,  like  a  blurred  star  shining 
through  a  mass  of  foggy  light. 

As  it  is  warmed  by  the  sun  signs  of  activity  become  manifest. 
The  tail  gradually  forms,  increasing  in  size  and  splendor  as  the 
comet  comes  nearer  the  sun.  The  nucleus  seems  to  be  in  ebulli- 
tion, throwing  off  masses  of  vapor,  or  ejecting  jets.  After  peri- 
helion passage  the  nucleus  gradually  becomes  fainter,  the  jets 
feebler,  the  head  larger,  and  the  tail  shorter,  until  the  comet  has 
reached  its  former  low  estate,  having  laid  aside  the  gay  trappings 
with  which  it  was  ornamented  at  perihelion. 

290.  Jets  and  Envelopes.  —  The  jets  or  fountains  of  matter  which 
spurt  out  from  the  nucleus  as  the  body  nears  perihelion  are  emitted 
from  the  sunward  side  of  the  nucleus,  and  are  directed  toward 
the  sun.  They  rise  higher  and  higher  and 
become  more  diffuse,  until  they  are  lost  in 
the  head.  One  is  exhibited  in  Fig.  150. 

A  well  behaved  nucleus  throws  off  en- 
velopes (Fig.  157).  These  rise  sunward,  one 
after  another,  becoming  fainter  and  more 
diffused  as  they  approach  the  outside  of  the 
head. 

291.  Tails :  their  Dimensions  and  Varieties. 
—  The  tail  is  by  far  the  bulkiest  part  of  a 
comet.  Tails  long  enough  to  reach  from  the 

earth  to  the  sun  are  not  uncommon.  The  extremity  of  such  a 
tail  is  millions  of  miles  in  thickness.  The  tail  of  the  comet  of 
1843  was  estimated  to  be  581,000000  miles  long  at  one  time.  A 
tail  a  tenth  as  long  as  this  is  reckoned  highly  respectable.  Some 
tails  are  narrow  and  straight,  like  prodigious  spines.  The  forms  of 
others  are  like  half-opened  fans.  A  comet  occasionally  has  several 
tails,  pointing  in  widely  different  directions. 

292.  Companion  Comets.  —  Comet  1889  V.  (Brooks)  was  very  re- 
markable on  account  of  the  group  of  companions  which  attended  it. 
There  were  four  of  these  comrades,  moving  along  a  little  in  advance 
of  the  main  comet. 

Two  of  the  companions  were  excessively  faint,  and  finally  disap- 


COMETS    AND    METEORS.  193 

peared.  The  two  brighter  ones  were  perfect  miniatures  of  the 
main  comet,  having  tiny  nuclei  and  shapely  tails.  But  their 
beauty  was  evanescent.  For  a  while  they  receded  from  the  prin- 
cipal comet.  In  three  weeks  the  nearer  companion  ceased  to 
recede;  it  then  enlarged,  became  very  diffuse,  and  disappeared 
completely,  as  if  blotted  out  of  existence.  The  farther  corn- 


Fig.  151.  —  COMPANIONS  OF  BROOKS'S  COMET. 

panion  continued  to  recede,  until  it  had  become  (a  month  after 
discovery)  brighter  than  the  main  comet.  In  a  month  more  it 
began  to  come  back  and  shed  its  tail ;  its  head  swelled  and  became 
diffuse  and  faint,  so  that  it  appeared  to  be  in  a  sorry  plight.  The 
companions  may  have  been  caused  by  a  disruption  of  the  parent 
mass  by  the  attraction  of  Jupiter  in  1886.  At  that  time  the  comet 
was  within  the  system  of  Jupiter's  satellites  for  over  two  days  and 
a  half,  and  may  have  been  struck  by  one  of  them  ;  it  may  even  have 

'3 


194  DESCRIPTIVE    ASTRONOMY. 

grazed  the  planet's  surface.  Companion  comets  are  not  very 
common. 

293.  Constitution  of  the  Head  and  of   the  Nucleus.  —  The   most 
plausible  theory  is  that  the  head  is  composed  of  a  mixture  of  solid 
and   gaseous    matter.     The  presence  of  gas  is  shown  by  the  spec- 
trum.    The  connection  known  to  exist  between  certain  comets  and 
meteors  (§  332)  renders  it  wellnigh  certain  tnat  solid  bodies  are 
scattered  throughout  the  head.     That  portions  of  the  solid  matter 
become  liquid  temporarily,  when  a  comet  like  that  of   1882  dashes 
through  the  sun's  corona,  is  almost  inevitable. 

The  size  of  the  solid  bodies  is  largely  a  matter  of  conjecture. 
Some  think  that  they  are  like  grains  of  sand ;  others  liken  them 
to  paving-stones  and  brick-bats.  The  nucleus  is  supposed  to  be 
the  densest  portion  of  this  swarm  of  meteoric  bodies.  The  nuclei 
of  some  large  comets  may  be  small  solid  bodies  of  great  density. 

294.  Evolution  of  the  Tail.  —  Comets'  tails  point  away  from  the 
sun,  except  in  a  few  rare  and  anomalous  instances.     The  material 

carried  up  by  the  jets  and  envelopes  seems 
to  be  repelled  by  the  sun,  and  driven 
away  from  it,  despite  its  gravitational  pull. 
The  nature  of  the  repulsive  force  is  un- 
known, but  it  is  generally  thought  to  be 
electrical.  Many  experiments  described 
in  works  on  physics  show  that  electrical 
attractions  and  repulsions,  acting  upon 
Fig.  152.  — DEVELOPMENT  light  bodies  which  have  a  large  surface  in 

OF   A   TAIL.  .  .,1         ,1  r  ,  -i 

comparison    with    their   mass,    frequently 

overcome  the  force  of  gravity.  A  body,  on  the  other  hand,  which 
has  a  small  surface,  but  considerable  weight,  obeys  the  force  of 
gravity.  So  the  lightest  portions  of  a  comet's  head  might  be 
driven  away  by  an  electrical  repulsion  originating  in  the  sun, 
while 'the  heavier  portions,  being  but  slightly  affected  by  this  re- 
pulsion, obeyed  the  law  of  gravitation.  There  is  evidence  that 
the  nucleus,  as  well  as  the  sun,  repels  the  finely  divided  matter. 
Photographs  of  Rordame's  comet,  taken  in  1893^  showed  that  cer- 
tain condensations  in  its  tail  were  receding  from  the  nucleus  at 
the  rate  of  fifty  miles  a  second. 

1  By  Prof.  W.  J.  Hussey,  at  the  Lick  Observatory. 


COMETS    AND    METEORS.  1 95 

295.  Types  of  Tails.  —  A  Russian  astronomer,  Bredichin,  has 
made  an  elaborate  investigation  of  the  forms  of  comets'  tails,  and 
has  divided  them  into  three  types. 

Tails  of  Type  I.  are  nearly  straight,  and  point  almost  directly 
away  from  the  sun.  They  are  thought  to  be  composed  of  hydrogen. 

SUNWARD  LINE 


Fig.  153.  —  TYPE  1. 

Type  II.  consists  of  the  gently  curving  trains  which  are  most 
common.  These  trains  are  probably  composed  of  compounds  of 
hydrogen  and  carbon,  known  to  chemists  as  hydro-carbons :  marsh 
gas  and  olefiant  gas  are  two  of  these. 


SUNWARD  LINE  ^ ^       SUNWARD    LINE 


Fig.  154. —  TYPE  II. 

Type  III.  is  not  common;  it  includes  short  tails  of  great  curva- 
ture. The  axes  of  such  tails  are  far  from  pointing  away  from  the 
sun.  Hence  they  are  composed  of  heavy  materials,  such  as  iron 
vapor. 

296.  Mass  and  Density.  —  The  masses  of  comets  must  be  very 
small  compared  with  those  of  the  major  planets;  otherwise  the 
earth  and  other  planets  would  have  been  disturbed  appreciably  by 
comets  which  have  come  near  to  them.  No  such  perturbation  has 
ever  been  manifest.  The  combined  mass  of  100,000  bodies,  each 
as  massive  as  one  of  the  greatest  comets,  would  not  equal  that  of 
the  earth.  Since  they  are  of  prodigious  size,  their  mean  density 
must  be  exceedingly  small.  Sir  John  Herschel  watched  a  comet 
as  it  passed  in  front  of  a  cluster  of  very  minute  stars ;  the  lustre  of 
the  stars  was  not  perceptibly  dimmed.  Even  when  a  star  is 
nearly  behind  the  nucleus  of  a  large  comet,  its  light  is  scarcely 
dimmed.  But  careful  observations  of  stars  which  were  shining 


196  DESCRIPTIVE    ASTRONOMY. 

through  hundreds  of  thousands  of  miles  of  cometary  matter  have 
shown  that  their  light  was  refracted  a  trifle  by  the  gases  of  the 
comet.  While  the  mean  density  of  a  comet  is  low,  the  density  of 
the  particles  of  solid  matter  which  make  up  most  of  its  mass  is 
probably  comparable  with  that  of  the  materials  composing  the 
earth's  crust.  Were  the  particles  closely  packed,  the  mean  density 
would  be  largely  increased.  The  tail  of  a  comet  is  so  tenuous  that 
it  may  be  appropriately  likened  to  "such  stuff  as  dreams  are 
made  ofc " 

297.  ^Xight  and  Spectra.  —  The  brightness  of  a  comet  generally 
varies  according  to  the  changes  of  its  distance  from  the  earth  and 
the  sun.     But  there  are  frequent  anomalous  variations  of  brilliancy 
which  would  not  take  place  were  the  light  merely  reflected  sun- 
light.    The  spectrum  of  a  comet  is  ordinarily  a  combination  of  two 
spectra:  one  is  a  faint  continuous  spectrum  due  to  reflected  sun- 
light; the  other  is  a  spectrum  of  bright  bands,  like  that  produced 
by   the  flame  of  a  Bunsen  burner.      It  is  due  to  glowing  hydro- 
carbon gases.      Comets  which  go  near  the  sun,  so  as  to  be  especially 
excited  by  its  influence,  exhibit  spectra  in  which  the  lines  due  to 
sodium  and  iron  are  plainly  visible. 

There  is  a  rapidly  accumulating  weight  of  evidence  in  favor  of 
the  theory  that  a  comet  derives  its  light,  except  that  portion  which 
is  merely  reflected  sunlight,  almost  wholly  from  electrical  dis- 
charges between  its  particles.  The  electrical  action  is  stimulated 
by  the  sun,  being  more  intense  the  nearer  the  comet  is  to  it. 

298.  Fate  of  Comets.  —  The  tail  of  a  comet  is  not  to  be  regarded 
as   the  same  to-day  that   it   was  yesterday.     When   one  of  those 
comets  which  dash  through  the  corona  goes  from  one  side  of  the 
sun  to  the  opposite  side  in  a  few  hours,  the  tail,  though  millions  of 
miles  in  length,  appears  also  to  be  swung  around,  like  a  gigantic 
scimetar  brandished  athwart  the  sky.      No  known  force  could  cause 
the  tail  to  swing  around  with  such  prodigious  velocity.      We  there- 
fore conclude  that  the  tail  resembles  the  cloud  of   smoke  puffed 
out   from   the  smoke-stack  of   a  locomotive.       Fresh  material    is 
driven  off  from  the  head  each  second,  to  form    the  tail.     As  the 
smoke  of  a  locomotive  does  not  return  to  it  again,  so  the  particles 
driven  off  from  the  comet  in  such  profusion  at  its  perihelion  pas- 
sage are  lost  in  space. 


COMETS    AND    METEORS.  1 97 

Thus  at  each  perihelion  passage  a  periodic  comet  loses  a  portion 
of  its  mass.  It  must  therefore  in  the  course  of  ages  be  much 
reduced  in  mass  and  brightness.  Sir  Isaac  Newton  expressed  the 
opinion  that  it  was  the  fate  of  comets  diffundi  tandem  et  spargi 
per coelos  universes,  —  "to  be  finally  diffused  and  scattered  through 
the  celestial  spaces." 

Comets  which  are  not  periodic,  and  which  therefore  do  not  visit 
the  sun  every  few  years,  may  in  their  infinite  journeyings  encounter 
other  suns,  swing  about  them  in  unwonted  splendor,  and  continue 
their  wanderings  till  they  are  captured  for  some  sun  by  the  aid  of 
one  of  its  planets,  and  are  then  gradually  shorn  of  their  beauty. 

299.    Superstitions.1  —  Milton  says,  in  the  second  book  of  Paradise 

Lost :  — 

"  On  the  other  side, 

Incensed  with  indignation,  Satan  stood 
Unterrified;  and  like  a  comet  burned, 
That  fires  the  length  of  Ophiuchus  huge 
In  the  arctic  sky,  and  from  his  horrid  hair 
Shakes  pestilence  and  war." 

Such  superstitions  have  been  rife  from  early  times.  Josephus 
mentions,  among  the  prodigies  which  foretold  the  destruction  of 
Jerusalem,  a  comet  with  a  tail  like  the  blade  of  a  sword,  which 
hung  over  the  city  a  year. 

The  Roman  Emperor  Vespasian,  when  nearing  his  end,  heard 
some  of  his  courtiers  conversing  in  a  low  tone  about  a  comet  then 
visible.  But  he  treated  the  matter  lightly,  saying,  "This  hairy 
star  does  not  concern  me;  it  menaces  rather  the  king  of  the 
Parthians,  for  he  is  hairy  and  I  am  bald." 

Throughout  the  Middle  Ages  comets  seem  to  have  been  regarded 
as  especially  presaging  the  death  of  kings. 

The  comet  afterward  known  as  Halley's  appeared  in  April, 
1066,  when  William  the  Conqueror  was  about  to  invade  England. 
"Nova  Stella,  novus  rex,"  was  the  proverb  of  the  time.  A  monk, 
apostrophizing  the  comet,  said :  "  Here  art  thou,  source  of  the 
tears  of  many  mothers.  Long  have  I  seen  thee;  but  now  thou 
appearest  to  me  more  terrible,  for  thou  menacest  my  country  with 
complete  ruin." 

1  See  the  English  edition  of  Guillemin's  "  World  of  Comets." 


198 


DESCRIPTIVE    ASTRONOMY. 


The  comet  of  1528  must  have  struck  terror  to  the  hearts  of  the 
beholders.  Ambrose  Pare,  one  of  the  most  learned  men  of  that 
time,  writes  of  it  as  follows :  — 

<•'  This  comet  was  so  horrible,  so  frightful,  and  it  produced  such  great 
terror  in  the  vulgar,  that  some  died  of  fear  and  others  fell  sick.  It  appeared 
to  be  of  excessive  length  and  was  of  the  color  of  blood.  At  the  summit  of 


Fig.  156.  —  COMET  OF  1528. 

it  was  seen  the  figure  of  a  bent  arm,  holding  in  its  hand  a  great  sword,  as  if 
about  to  strike.  At  the  end  of  the  point  there  were  three  stars.  On  both 
sides  of  the  rays  of  this  comet  were  seen  a  great  number  of  axes,  knives, 
blood-colored  swords,  among  which  were  a  great  number  of  hideous  human 
faces,  with  beards  and  bristling  hair." 


UNIVERSITY 
COMETS    AND    METEORS^^CALIFORjj^^     199 

Through  the  instrumentality  of  modern  science  the  terror 
formerly  inspired  by  great  comets  has  largely  given  place  to  a 
lively  delight  in  watching  their  beautiful  forms  and  wonderful 
changes. 

300.  Collisions.  —  Comets  are  still  feared  by  many  people,  on  the 
ground  that  they  may  collide  with  the  earth  and  arrest  its  motion, 
so  that  it  will  begin  to  fall  toward  the  sun,  or  that  they  may 
produce  such  intense  heat  by  the  impact  that,  in  the  words  of 
Prospero,  — 

"  The  great  globe  itself, 
Yea,  all  which  it  inherit,  shall  dissolve, 
And,  like  this  insubstantial  pageant  faded, 
Leave  not  a  rack  behind." 

From  what  we  have  already  learned  concerning  the  masses  of 
comets,  it  is  plain  that  there  is  no  ground  for  apprehension  that 
the  earth's  orbit  would  be  much  changed  by  collision  with  even 
the  largest  of  them.  Computations  of  the  orbit  of  the  magnificent 
comet  of  1 86 1  (Fig.  157)  showed  that  the  earth  probably  passed 
through  its  tail  on  a  certain  night.  The  result  was  no  more 
serious  than  if  our  planet  had  been  smitten  by  the  club  of  a 
phantom.  If  the  earth  encountered  the  head  of  an  ordinary 
comet,  the  meteoric  masses  of  which  it  is  presumably  composed 
might  be  dissipated  into  vapor  when  they  struck  the  atmosphere. 

Should  these  particles  prove  to  be  metallic  masses  as  large  as 
the  fist,  able  to  plough  through  the  atmosphere,  and  to  make  a 
fiery  rain  upon  the  earth's  surface,  the  bombardment  would  be 
memorable. 

While  there  are  no  very  definite  data  to  reason  from,  it  is  be- 
lieved that  an  encounter  with  the  nucleus  of  one  of  the  largest 
comets  is  not  to  be  desired. 

So  vast  are  the  celestial  spaces  in  comparison  with  the  bodies  by 
which  they  are  peopled,  that  the  chance  that  the  earth  will  strike 
a  good-sized  comet  some  time  during  the  next  100,000  years  is 
exceedingly  slight. 

Most  astronomers  would  be  delighted  with  the  prospect  that  the 
earth  was  going  to  blaze  a  pathway  through  some  ordinary  comet. 
They  would  also  be  pleased  to  watch  some  large  comet  dashing 


200 


DESCRIPTIVE    ASTRONOMY. 


headlong   into   the   sun.      Professor   Young   thinks   that   the  heat 
evolved    by   the    collision   would    be    chiefly  liberated   below   the 


Fig.  157.  —  COMET  OF  1861. 

solar  surface,  and  would  simply  add  a  trifle  to  the  sun's  store  of 
energy. 

In   November,  1892,  when   it  was   supposed  that   Biela's  comet 
was  about  to  strike  the  earth,  there  was  considerable  fright.      The 


COMETS    AND    METEORS.  2OI 

following  despatch  from  Atlanta,  Georgia,  was  printed  in  a  daily 
paper. 

"  The  fear  which  took  possession  of  many  citizens  has  not  yet  abated. 
The  general  expectation  hereabouts  was  that  the  comet  would  be  heard 
from  on  Saturday  night.  As  one  result,  the  confessionals  of  the  two 
Catholic  churches  here  were  crowded  yesterday  evening.  As  the  night 
advanced  there  were  many  who  insisted  that  they  could  detect  a  change 
in  the  atmosphere.  The  air,  they  said,  was  stifling.  It  was  wonderful  to 
see  how  many  persons  gathered  from  different  sections  of  the  city  around 
the  newspaper  offices  with  substantially  the  same  statement.  As  a  conse- 
quence, many  families  of  the  better  class  kept  watch  all  night,  in  order  that 
if  the  worst  came  they  might  be  awake  to  meet  it.  The  orgies  around  the 
colored  churches  would  be  laughable,  were  it  not  for  the  seriousness  with 
which  the  worshippers  take  the  matter.  To-night  (Saturday)  they  are  all 
full,  and  sermons  suited  to  the  terrible  occasion  are  being  delivered." 

REMARKABLE    COMETS. 

301.  Halley's  Comet.  —  Halley,  who  was  a  contemporary  of  New- 
ton, having  learned  that,  according  to  the  recently  propounded 
theory  of  gravitation,  comets  might  move  in  elliptic  orbits,  and 
thus  be  visible  at  several  returns,  computed  the  orbits  of  two  dozen 
comets.  On  comparing  the  computations  he  observed  that  the 
orbits  of  the  comets  of  1531,  1607,  and  1682  were  strikingly  simi- 
lar; he  reasoned  from  this  that  these  three  were  one  and  the  same 
body,  revolving  about  the  sun  in  about  seventy-five  years.  He 
predicted  its  return  about  1758;  knowing  that  he  would  be  in  his 
grave  before  that  time,  he  expressed  a  modest  hope  that,  if  the 
comet  should  return  then,  "posterity  would  not  refuse  to  acknowl- 
edge that  this  was  discovered  by  an  Englishman."  In  1757 
astronomers  began  to  watch  for  it.  For  weary  months  the  quest 
was  vain.  Clairaut,  a  French  mathematician,  showed  by  an  elab- 
orate investigation,  which  challenged  the  admiration  of  the  world, 
that  the  comet  would  be  retarded  618  days  by  the  attraction  of 
Jupiter  and  Saturn,  and  would  arrive  at  perihelion  in  the  middle 
of  April,  1759.  He  said  that  this  might  be  a  month  in  error.  The 
comet  was  first  seen  on  Christmas  night,  1758,  and  arrived  at 
perihelion  on  March  13,  a  month  before  the  time  set.  Thus  the 
Newtonian  principle  of  gravitation  received  a  striking  verification. 


2O2  DESCRIPTIVE    ASTRONOMY. 

This  comet  was  seen  many  times  before  Halley's  day.  Its 
earliest  recorded  appearance  is  supposed  to  be  that  of  B.  C.  1 1.  It 
is  expected  again  in  1910  or  1911. 

302.  Encke's  Comet.  —  This  comet  is  insignificant  in  appearance, 
rarely  exhibiting  a  sharply  defined  nucleus,  and  sporting  the  scan- 
tiest of  tails.      It  was  discovered  in  1786,  and  thirty-two  years  after- 
wards was  found  to  be  revolving   in  a  period  of  only  three  years 
and  a  quarter,  the  shortest  known  period.     It  is  specially  interest- 
ing because  its  motion  suffers  much  at  the  hands  of  Mercury,  and 
is  also  accelerated  in  a  strange  way.     Encke's  laborious  computa- 
tions showed  that  its  periodic  time  was  shortened  nearly  three  hours 
at  each  revolution.     More  recent  observations  and  discussions  show 
a  reduction  of  the  acceleration  to  one  half  its  former  value.     The 
acceleration  was  at  one  time  considered  a  triumphant  proof  of  the 
existence  in  space  of  a  resisting  medium,  the  luminiferous  ether. 
Such  a  medium,  by  retarding  the  comet's  motion,  would  cause  it 
to  drop  toward  the  sun,  and  revolve  in  a  smaller  orbit,  in  which  it 
would  move  with  greater  speed.      But  the  recent  diminution  of  the 
acceleration  does  not  bear  out  this  theory.     The  comet's  disturbance 
may  be  due  to  collisions  with  small  bodies  coming  across  its  path, 
or  to  its  own  feeble  activities  in  the  line  of  throwing  off  envelopes 
or  jets. 

Its  perturbations  by  Mercury  afford  a  means  of  determining  the 
mass  of  that  planet. 

303.  Biela'i,  JJpmet.  —  This  was  discovered  by  Biela,  an  Austrian, 
in  1826;  its  periodic  time  was  computed  to  be  6^  years.      It  was 
due  again  in   1832,  and  then  gave  rise  to  the  first  comet  scare  of 
this  century.     For  calculation  showed  that  it  came  close  to  the 
earth's  orbit,  and   ignorant   people  became  much   alarmed  at  the 
prospect  of  a  collision.      Though  it  came  near  the  earth's  orbit, 
the  earth  itself  was  at  no  time  nearer  to  it  than   15,000000  miles. 
In  December,    1845,   it  was  found  to  be  elongated,   and  ten  days 
afterward  it  split  into  two  comets,  one  of  them  being  much  smaller 
than  the  other.      Both  became  brighter  and  developed  well  defined 
nuclei    and    tails.      The    smaller   one  grew  in  brightness  until   it 
surpassed   the    other   for  a  time.       Their   distance   apart    became 
157,000  miles,  and  they  were  lost  to  view  in  April,  1846. 

In  1852  the  twins  were  seen  again;  the  distance  between  them 


COMETS    AND    METEORS.  203 

had  increased  to  a  million  and  a  half  miles;  sometimes  one  was  the 
brighter,  sometimes  the  other.  They  faded  from  sight  again,  and 
have  never  been  seen  since,  though  they  were  in  a  very  favorable 
position  in  1872.  Comet  Holmes,  which  appeared  in  November, 
1892,  was  at  first  thought  to  be  the  long  lost  Biela,  but  the  compu- 
tation of  its  orbit  showed  that  this  was  not  the  case. 

304.  The  Great  Comet  of  1882.  —  This  was  discovered  early  in 
September  by  observers  in  the  southern  hemisphere.  On  Septem- 
ber 17,  the  observers  at  the  Cape  of  Good  Hope  saw  this  aston- 
ishingly brilliant  body  move  swiftly  up  to  the  limb  of  the  sun, 


Fig.  158.  —  THE  GREAT  COMET  OF  1882. 

and  then  vanish  completely  as  it  swept  over  its  broad  disk.  On 
the  following  day  it  was  seen  close  to  the  sun  by  observers  all  over 
the  world.  It  was  only  necessary  to  screen  off  the  sun's  light  by 
holding  up  the  hand  at  arm's  length  in  order  to  see  the  comet. 
In  less  than  a  week  the  nucleus,  formerly  round,  became  oval,  and 
by  the  end  of  a  month  two  centres  of  condensation  were  seen. 
During  the  next  three  months  the  nucleus  became  divided  into 
four  or  five  condensations,  ranged  in  a  line  and  connected  by  a  hazy 
mass  of  light. 

Meanwhile  a  magnificent  train,  100,000000  miles  in  length,  had 
been  developed.      More  than  half  a  dozen  companion  comets  were 


2O4 


DESCRIPTIVE    ASTRONOMY. 


seen,  all  evanescent.  The  fiery  object  seemed  to  be  strewing  its 
path  with  filmy  debris,  thrown  off  by  some  unknown  force.  Possi- 
bly they  were  fragments  driven  off  by  the  intense  repulsive  action 
of  the  sun,  as  the  comet  dashed  through  the  corona.  When  at 
perihelion,  it  was  less  than  300,000  miles  from  the  sun's  surface. 

There   was   a   faint  but  prodigious  sheath   of  cometary   matter 
which  enveloped  most  of  the  comet,  and  projected  millions  of  miles 


Fig.  159.  —  NUCLEUS  OF  THE  GREAT  COMET  OF  1882,  AT  DIFFERENT  TIMES. 

in  front  of  the  head.  The  orbit  was  not  appreciably  changed  by 
any  resistance  due  to  the  coronal  matter.  The  periodic  time  is  772 
years. 

Besides  the  hydro-carbons  ordinarily  found  in  comets,  there  were 
in  this  body  sodium  and  iron;  some  of  the  numerous  bright  lines 
in  the  spectrum  were  probably  due  to  calcium  and  manganese. 

305.  Swift's  l  Bright  Comet  of  1892.  —  This  is  mentioned  on  ac- 
count  of  the  marvellous  changes  observed  in  its  tail.  On  April  4 
it  was  20°  long,  straight  and  slender;  in  the  telescope  it  was  seen 
to  consist  of  two  branches,  between  which  scarcely  any  cometary 
matter  was  visible.  The  next  morning  a  new  tail  had  formed 
between  the  other  two,  and  each  tail  was  composed  of  several 
lying  close  together.  At  least  a  dozen  could  be  counted.  After 
the  lapse  of  another  day,  one  of  the  original  three  tails  had  van- 
ished, and  the  other  two  were  blended. 

Then  one  of  these  grew  bright,  and  the  other  faded  away;  the 
bright  one  had  a  sharp  bend  in  it,  as  if  turned  aside  by  some 


1  Lewis  Swift,  of  Echo  Mountain,  California. 


COMETS    AND    METEORS^ 


2O5 


Fig.  160.  —  SWIFT'S  COMET:   PHOTOGRAPHED  BY  BARNARD. 


2O6  DESCRIPTIVE    ASTRONOMY. 

obstacle.  Near  the  point  of  deflection  were  two  dark  spots  in  the 
brightest  part  of  the  tail.  Finally  the  tail  split  up  into  six 
branches.  All  these  changes  and  some  others  took  place  in  five 
days. 

306.  Comet  Holmes. — This  was  discovered  on  Nov.  6,  1892,  by 
Mr.  Holmes,  an  English  amateur  astronomer.      Its  position  in  the 
sky  was  near  that  in  which  Biela's  comet  might  appear,  and  the 
latter  (if  still  in   existence)  was  due.     Therefore  the  comet  was 
supposed  by  some  to  be  Biela's,  and  preliminary  computations  led 
to  the  interesting  result  that  the  comet  was  likely  to  collide  with  the 
earth.      On  this  account  a  wide-spread  popular  interest  was  awak- 
ened (§  303).     The  comet  changed  its  position  in  the  sky  so  little, 
for  several  weeks,  that  its  orbit  could  not  be  computed  with  much 
accuracy  at    first.      It  was  finally  found  to  be  moving  in  a  small 
ellipse,  which  is  more  nearly  circular  than  the  orbit  of  any  other 
known  comet;   the  period  is  less   than    seven    years.     The   orbit 
resembles  that  of  an  asteroid.     The  comet,  which  was  at  first  vis- 
ible to  the  naked  eye,  grew  faint  and  diffuse  in  a  few  weeks.      But 
in  the  middle  of  January,  1893,  it  suffered  a  strange  transformation, 
changing    into   a   starlike    object    surrounded   by  a   small  circular 
nebulosity.      On  January  16,  Dr.  Barnard  saw  the  nucleus  brighten 
considerably  while  he  was  observing  it.      During  the  next  week  the 
nucleus  suffered  marked  changes  of  brightness,  being  very  plain  at 
times,  and  almost  invisible  at  others.      The  surrounding  nebulosity 
soon  grew  larger  and  fainter,  and  faded  away. 

307.  Comet  c  1893  (Brooks).  —  This  comet  was  discovered  on  the 
morning  of  Oct.  17,  1893,  by  William  R,  Brooks,  of  Geneva,  N.Y. 
At  first  it  had  a  short  tail  issuing  from  the  northern  side  of  the 
head,    in  addition  to  the  main  tail,  which  was  straight  and  of  a 
graceful  form.      But  a  photograph  taken  by  Dr.  E.  E.  Barnard  on 
the   morning   of    October  21    (October  20  by  astronomical    time) 
showed   remarkable  changes  in  the  tail,   which  Dr.    Barnard  thus 
describes.1 

"  It  presented  the  comet's  tail  as  no  comet's  tail  was  ever  seen  before. 
The  graceful  symmetry  was  destroyed ;  the  tail  was  shattered.  It  was 
bent,  distorted,  and  deflected,  while  the  larger  part  of  it  was  broken  up 

1  Popular  Astronomy,  December,  1893. 


COMETS    AND    METEORS.  2O/ 

into  knots  and  masses  of  nebulosity,  the  whole  appearance  giving  the 
idea  of  a  torch  flickering  and  streaming  irregularly  in  the  wind.  The 
short  northern  tail  was  swept  entirely  away,  and  the  comet  itself  was 
much  brighter. 

"  The  very  appearance  at  once  suggested  an  explanation,  which  is  prob- 
ably the  true  one.  If  the  comet's  tail,  in  its  flight  through  space,  had 
suddenly  encountered  a  resisting  medium  which  had  passed  through  the 
tail  near  the  middle,  we  should  have  precisely  the  appearance  presented 
by  the  comet.  It  is  not  necessary  that  the  medium  should  be  a  solid 
body ;  if  it  possessed  only  the  feeblest  of  ethereal  lightness  it  would  de- 
flect, distort,  and  shatter  the  tail.  What  makes  this  explanation  all  the 
more  probable  is  that  the  disturbance  was  produced  from  the  side  of 
the  tail  that  was  advancing  through  space." 


Fig.  161.  —  BROOKS'S  COMET:  PHOTOGRAPHED  BY  BARNARD. 

The  appearance  may  also  be  explained  by  variations  in  the 
amount  and  in  the  direction  of  motion  of  matter  driven  off  from 
the  comet's  head. 

METEORS. 

308.  The  Two  Classes.  —  Meteors  are  divided  into  two  classes, 
meteorites  and  shooting  stars.  Meteorites  are  the  bright  bodies 
which  from  time  to  time  dash  through  the  air,  like  balls  of  fire, 
and  fall  to  the  ground.  There  are  various  other  names  for  them, 
the  most  common  one  being  aerolites.  Brilliant  meteoric  objects 
which  do  not  fall  to  the  earth  are  ordinarily  designated  only  by  the 
general  term  "meteor."  Shooting  stars,  on  the  other  hand,  are 
less  conspicuous  bodies,  which  can  be  seen  on  any  clear  dark 


2O8  DESCRIPTIVE    ASTRONOMY. 

night,   darting   across    the   sky;    they  usually  attract  no   especial 
attention  by  their  brightness,  and  never  fall  to  the  earth. 

309.  Past  Appearances  of  Meteorites.  —  Falls  of    meteorites   are 
recorded  before  the  present  era.     Though  there  are  many  records 
of  the  falling  of  stones  from  the  sky,  they  were  at  the  close  of  the 
last  century  regarded  by  most  scientific  men  as  old  wives'  fables. 

There  is  one  at  Mecca,  which  is  adored  by  the  faithful  Mussul- 
man. An  Emperor  in  the  Middle  Ages  was  said  to  have  a  sword 
which  was  fashioned  from  one  of  these  celestial  visitors.  In  April, 
1889,  copper  earrings  plated  with  meteoric  iron  were  found  in  an 
Indian  mound  in  Ohio.  During  the  last  decade  of  the  eighteenth 
century,  several  falls  of  meteorites  occurred,  which  were  so  reli- 
ably substantiated  that  scientific  men  began  to  inquire  into  the 
matter  earnestly. 

In  1803  such  a  shower  fell  at  L'Aigle  in  Normandy,  that  the 
French  Academy  sent  one  of  their  number  to  inquire  into  the 
matter.  So  exhaustive  was  his  investigation,  and  so  convincing 
his  report,  that  the  most  sceptical  were  forced  to  admit  that  stones 
fell  from  the  sky.  The  story  of  one  of  the  best  authenticated  as 
well  as  most  remarkable  of  meteorites  is  told  in  the  next  section. 

310.  The  Ensisheim   Meteorite.  —  This  meteorite  fell    on    Nov.   7, 
1492,  at  Ensisheim,  in  Alsace,  and  an  account  of  it,  together  with 
the  stone  itself,  was  put  in  the  church  at  that  place.     The  account 
states  that  on  the  day  in  question,  some  minutes  before  noon,  there 
was  a  loud  noise,  like  the  rumbling  of  thunder,  and  a  stone  weighing 
280  Ibs.  was  seen  by  a  child  to  dash  into  a  ploughed  field  and  bury 
itself  about  three  feet  in  the  earth.     Some  small  pieces  of  it  were 
taken  for  examination,  but  the  parent  mass  was  suspended  in  the 
choir  of  the  church.    There  it  remained  until  it  was  ruthlessly  taken 
away  during  the   French  Revolution.     It  has   since  been   restored 
to  the  town  hall  of  the  village. 

311.  A  Detonating  Meteor.  —  On  Dec.  21,  1876,  a  superb   fireball 
appeared  over  the  State  of  Kansas,  and  moved  thence  eastward  south 
of  Chicago  across  Indiana,  over  Lake  Erie,  to  Lake  Ontario,  where 
it  disappeared.     When  nearly  200  miles  from  Bloomington,  Indiana, 
the  meteor  burst,  and  the  inhabitants  of  that  city  saw  a  magnificent 
array  of  fireballs  sweeping  through  the  evening  sky.     After  the  ex- 
citement aroused  by  the  marvellous  spectacle  was  over,  there  came  a 


COMETS    AND    METEORS.  2OQ 

tremendous  crash,  like  the  heaviest  reverberations  of  thunder.  The 
concussion  which  accompanied  it  led  some  to  think  that  a  light 
earthquake  had  shaken  the  town.  How  terrific  must  a  detonation 
have  been,  which  was  so  startling  nearly  200  miles  away,  after  the 
sound  waves  had  been  on  their  journey  a  quarter  of  an  hour ! 

312.  Kiowa   County    (Kansas)    Meteorites.  —  In    March,    1890,  the 
attention  of  scientific  men  was  called  to  several  strange  pieces  of 
iron,  which  had  been  ploughed  up  from  time  to  time  in  Brenham 
township,  Kiowa  County,  Kansas.     They  had  been  put  to  various 
uses,  such  as  holding  down  the  cover  of  a  rain  barrel,  and  keeping 
the  roof  of  a  stable  from  blowing  away,  and  helping  to  stop  a  fence 
hole  through  which  hogs  escaped  from  their  feeding  ground.     One 
had  risen  to   the  dignity  of  ornamenting  the  sidewalk  in  front  of  a 
real  estate  office. 

A  cowboy  and  a  woman  were  the  only  people  in  the  vicinity  who 
seemed  to  realize  the  value  of  these  articles.  The  cowboy,  being 
unable  to  carry  his  off,  buried  them,  and  died  shortly  afterward.  The 
woman  sent  for  a  college  professor,  and  disposed  of  hers  for  enough 
to  pay  off  the  mortgage  on  her  farm. 

The  meteorites  were  found  scattered  over  an  oval  area  about  a 
mile  long.  The  largest  mass,  called  by  the  farmers  the  "  moon  me- 
teorite," weighed  466  pounds.  The  total  weight  of  the  twenty  odd 
pieces  found  was  about  a  ton. 

313.  A  Meteorite  in  Flight.  —  A  flying  meteorite  is  an  object  of 
dazzling  splendor,  when  seen  by  night.     It  is  generally  followed  by 
a  luminous  train,  which  may  remain  for  some  minutes.     A  noise  like 
the   roar  of  artillery  is   heard,  with   occasional  crashes   like  those 
caused  by  the  explosion  of  shells,  which  signalize  the  breaking  of 
the  main  body. 

When  the  meteorite  is  many  miles  away,  one  may  see  a  flash  of 
light  without  hearing  the  accompanying  detonation.  In  a  very  few 
seconds  all  is  over,  save  that  there  is  a  bright  irregular  streak  of 
light  on  the  sky,  marking  the  meteor's  path.  A  recent  meteorite  is 
reported  as  having  the  distinct  shape  of  a  banana,  and  as  turning  end 
for  end  as  it  flew,  scattering  sparks  along  its  pathway:  it  glowed 
like  a  piece  of  red-hot  iron,  though  the  sun  was  half  an  hour  above 
the  horizon. 

The  meteor's  velocity  is  so  effectually  checked  by  the  resistance 

14 


2IO  DESCRIPTIVE    ASTRONOMY. 

of  the  air,  that  it  finally  comes  to  earth  like  a  spent  cannon  ball  or 
a  shower  of  grape. 

314.  Path  and  Velocity.  —  Meteorites  move  in  orbits  about  the 
sun :  ninety  per  cent  of  those  orbits  which  have  been  computed  are 
elliptical.  These  bodies  generally  become  inflamed  at  a  height  of 
eighty  miles  or  more  above  the  earth's  surface,  despite  the  rarity  of 
the  air  at  that  elevation.  The  path  is  occasionally  hundreds  of  miles 


Fig.  162.  —  METEOR  SEEN  AT   BASSEIN,  BURMAH. 
(From  WinchdFs  "  World  Life,"  by  permission.) 

in  length.  The  velocity  of  a  meteorite  with  reference  to  the  earth 
depends  upon  the  relative  directions  of  their  motions.  When  the 
collision  occurs  "  head  on"  the  relative  velocity  exceeds  forty  miles 
a  second.  When  the  meteor  catches  up  with  the  earth,  both  being 
in  motion  in  the  same  direction,  the  relative  velocity  sinks  below  ten 
miles  a  second.  The  velocity  with  which  the  meteor  reaches  the 
ground  is  often  considerably  less  than  that  of  a  cannon  ball,  as  is 
shown  by  the  slight  depth  to  which  it  penetrates.  Notwithstanding 
this  comparatively  low  velocity,  a  meteorite  is  no  insignificant  mis- 
sile. There  are  a  few  records  of  the  destruction  of  buildings  by 
them.  The  Chinese,  not  to  be  outdone  by  Western  nations,  have  a 
record  of  one  which  came  to  earth  2,500  years  ago,  destroying 
several  chariots  and  killing  ten  men. 


COMETS    AND    METEORS.  2  I  I 

Though  the  meteorite  which  flew  into  fragments  over  Madrid 
on  Feb.  10,  1896,  was  about  fifteen  miles  above  the  city,  the  con- 
cussion caused  strong  vibrations  of  partitions  of  houses,  and  exten- 
sive damage  to  windows. 

315.  Cause  of  Light  and  Heat.  — When  the  motion  of  a  cannon  ball 
is  arrested  by  striking  an  armor  plate,  the  ball  and  the  plate  are 
heated,  so  that  the  armor  plate    becomes  viscous  at  the  point  of 
striking,  and  flows  like  tar.     The  energy  possessed  by  the  ball  be- 
cause of  the  swiftness  of  its  motion  is  transformed  into  heat  when 
the   motion  is  arrested.      By  the  same  principle   a   nail  is  heated, 
when  struck   repeatedly  by  a  hammer.      The  energy  in  the  mov- 
ing hammer  is  changed  into  heat.     As  the  speed  of  a  rifle  ball  is 
checked    when   it    is   fired    into  water,    the  speed    of  a    meteorite, 
which  may  be  one  hundred  times  as  great  as  that  of  the  rifle  ball, 
suffers    disastrous    diminution    even    in    the    upper    regions   of  the 
atmosphere,  and  is  almost  destroyed  before  the  body  reaches  the 
ground.     The  energy  lost  by  the  meteor,  as  its  speed  diminishes, 
reappears  in  heat.     The  air  is  heated  enormously ;  the  quantity  of 
heat  developed,  if  it  were  all  spent  on  the  meteorite,  would  liquefy 
it,  were    it   of  iron.     The    meteor    shines,    because    its    surface   is 
intensely  hot:    most  of  the  light  which  we  see  undoubtedly  comes 
from  the  incandescent   gases  surrounding  the   meteor.     The  train 
left   behind    remains    luminous    for  so    long  a  time   that   its   light 
cannot  be  accounted  for  by  heat  alone :    it  may  possibly  be  due 
to  phosphorescence. 

316.  Effect  of  the  Heat.  —  If  a  candle  were  thrown  through  the 
flames  of  a  large  bonfire,  some  of  its  surface  would  be  melted  off, 
but  the  interior  of  the  candle  might  not  be  heated  perceptibly.     In 
like    manner,   a  meteor   when    dashing   through   the    heated  air  is 
affected  as  if  it  were  passing  through  a  sea  of  burning  gas  raging 
with   uncontrollable  fury.     The  outside  of  the  meteor  is  fused  at 
once,  and  wiped  off  to  form  the  train.     As  the  exterior  is  very  hot, 
the  meteorite  is  liable  to  crack,  and  strew  its  path  with  its  own 
debris.     Sometimes   the  heating   causes  a  terrific    explosion,  or  a 
series  of  explosions,  which  reduce  the  meteor  to  small  Tragments. 
So   rapid  is  the  entire  process   that   the    heat  at  times  does  not 
penetrate  to  the  interior  of  the  mass.     While  most  pieces  of  freshly 
fallen  meteorites  are  too  hot  to  handle,  some  are  cold.     A  portion 


212  DESCRIPTIVE    ASTRONOMY. 

of  one  which  fell  in  India  was  found,  about  half  an  hour  afterward, 
embedded  in  moist  earth,  and  coated  with  ice. 

The  intensity  of  the  heat  to  which  a  meteor  is  exposed  may  be 
illustrated  by  the  case  of  a  fireball  which  was  observed  in  England 
in  1869.  The  luminous  fiery  envelope  was  more  than  four  miles  in 
diameter,  and  the  entire  meteor  was  vaporized  in  five  seconds,  while 
travelling  170  miles.  There  remained  a  cloud  of  glowing  vapor 
about  fifty  miles  long  and  four  miles  broad,  which  was  visible  for 
fifty  minutes. 

317.  Meteoric  Stones.  —  Most  of  the  meteoric  masses  which  fall  to 
the  earth  are  of  a  stony  nature.     When  found  they  are  glazed  over 
with  a  thin  crust  formed  by  the  fusion  of  the  exterior  during  the 
flight.      When   a  meteorite  bursts   just   |^fore  falling,   the   freshly 
broken  surfaces  are  not  thus  incrusted,  and  the  pieces  have  in  some 
cases  been  fitted  together  again.     The  surface  is  usually  indented 
with  numerous  pits  caused  by  the  fusion  of  parts  of  the  meteoric 
mass.     The  structure  of  some  of  these  objects  is  like  that  of  certain 
volcanic  rocks,  which  are  formed  of  irregular  masses  of  various  ma- 
terials held  together  by  a  cement.     In  half  a  dozen  meteorites  com- 
pounds of  carbon  have  been  found,  which  are  like  those  resulting 
from  the  decay  of  vegetable  life ;   but  no  forms  of  vegetation  such  as 
we  frequently  find  in  terrestrial  sandstones  have  been  discovered. 

318.  Meteoric  Iron :    Intermediate  Forms.  —  A  small  percentage  of 
meteorites  are  composed  of  iron,  which  is  alloyed  with  nickel  in  all 

yet  analyzed.  Only  about  a 
dozen  of  these  have  actually 
been  seen  to  fall.  The  others 
have  been  found  lying  on  or 
near  the  surface,  in  places 
where  there  is  no  other  iron  in 
the  vicinity.  A  few  of  these 
masses  weighed  several  tons. 

There  are  forms   intermedi- 
ate between  meteoric  iron  and 
Fig.  163.  — A  METEORITE.  stone.     In  some  of  these  there 

(From  "  Science,"  by  permission.)  .  , 

is  a  honeycombed  mass  of  iron, 

the  cavities  in  which  are  filled  with  various  minerals.     In  others  bits 
of  iron  are  found  scattered  through  a  stony  mass. 


COMETS    AND    METEORS. 


213 


319.  Elements  found  in  Meteorites.  —  More  than  a  third  of  the 
known  chemical  elements  are  found  in  meteorites ;  no  new  element 
has  been  discovered  in  them,  but  some  new  compounds  have  been 
found.  Most  of  the  elements  are  common  ones,  such  as  sulphur, 
phosphorus,  copper,  tin,  aluminum,  calcium,  etc.  There  are,  as 


Fig.  164.  —  THE  CANYON  DIABLO  METEORITE. 


yet,  no  traces  of  gold  or  silver.  Small  black  diamonds,  and  also 
minute  white  ones,  have  been  found  in  cavities  of  meteoric  iron, 
which  came  from  the  Canyon  Diablo,  in  Arizona,  where  about 
three  hundred  fragments  of  meteoric  iron  were  discovered  in  1891  : 
the  largest  piece,  weighing  1,015  pounds,  was  on  exhibition  at  the 
World's  Fair  in  Chicago.  Dust  from  the  diamonds  was  employed 
in  the  Tiffany  pavilion  at  the  Fair,  to  polish  other  diamonds :  the 
success  of  the  experiment  proved  conclusively  that  the  hard  black 
grains  from  the  meteorite  were  genuine  diamonds. 

320.  Origin.  —  The  Austrian  mineralogist,  Tschermak,  after  a  care- 
ful study  of  the  structure  of  meteorites,  reached"  the  conclusion  that 
they  had  a  volcanic  origin.  The  volcanoes  of  the  moon  are  now  ex- 
tinct. No  terrestrial  one  has  sufficient  power  to  eject  a  missile  with 


214  DESCRIPTIVE    ASTRONOMY. 

a  velocity  of  six  miles  a  second,  which  would  be  necessary  to  carry 
it  away  from  the  earth's  attraction,  if  the  earth  had  no  atmosphere, 
and  to  render  it  obedient  to  the  sun.  But  when  these  bodies  or 
other  planets  were  young,  their  volcanoes  may  have  possessed  the 
necessary  energy.  The  sun,  as  we  have  mentioned  (§  77),  has  been 
seen  to  eject  masses  of  heated  vapor  with  such  a  velocity  that  they 
would  not  fall  back  again.  Such  masses,  exposed  to  the  cold  of 
space,  would  be  condensed  into  solid  bodies,  like  the  meteorites. 
It  has  been  the  general  belief  among  astronomers  that  meteorites 
originated  like  the  large  heavenly  bodies,  from  the  condensation  of 
nebulous  matter  scattered  throughout  the  universe.  But  Tscher- 
mak's  theory  is  considered  worthy  of  careful  examination. 

321.  How  to  Observe  a  Meteorite.  —  The  observations  necessary  for 
determining  a  meteor's  path  can  be  easily  made  by  any  intelligent 
person  who  gets  a  fair  view  of  it.     Two  things  are  to  be  observed, 
the  time  and  the  direction  of  movement. 

At  night  one  familiar  with  the  constellations  may  note  its  path 
among  the  stars,  and  the  time  at  which  it  disappears.  By  repeating 
"  Mary  had  a  little  lamb,"  etc.,  beginning  when  the  meteor  is  first 
seen  and  stopping  at  its  disappearance,  the  length  of  time  during 
which  it  is  visible  can  be  estimated ;  for  the  number  of  seconds 
required  to  repeat  the  same  snatch  of  rhyme  is  easily  obtained 
afterwards  by  rehearsing  it,  watch  in  hand.  In  the  daytime,  the 
position  of  the  body  when  first  and  last  seen  can  be  noted  with  refer- 
ence to  surrounding  objects.  The  altitude  and  azimuth  (§  1 21)  of 
the  object  at  each  of  these  times  can  be  measured  later  with  a  sur- 
veyor's transit,  placed  at  the  point  where  the  observer  stood.1 

SHOOTING  STARS. 

322.  Numbers.  —  One  can  scarcely  look  at  the  face  of  the  sky  for 
fifteen  minutes  on  a  clear  moonless  night  without  seeing  at  least  one 
of  these  objects   dart  harmlessly  across  the  sky,  and  disappear  in 
silence,  leaving  no  trace  behind.     It  has  been  estimated  that  if  ob- 

1  A  careful  report  of  such  observations,  however  rude  they  may  seem  to  the  ob- 
server, would  be  welcomed  by  any  astronomical  periodical.  Such  communications  may 
be  sent  to  the  "Astronomical  Journal,"  Cambridge,  Mass.,  or  to  "  Popular  Astronomy," 
Northfield,  Minn. 


COMETS    AND    METEORS. 


215 


servers  were  uniformly  distributed  over  the  earth,  in  such  a  way  as 
to  command  a  view  of  all  the  shooting  stars  entering  the  atmosphere, 
ten  or  fifteen  million  of  them  would  be  found  to  strike  the  earth 
during  a  day.  *; 

They  are  twice  as  frequent  at  6  A.  M.  as  at  6  P.  M.  For  in  the 
morning  we  are  in  front  of  the  earth,  as  it  moves  in  its  orbit,  while  in 
the  evening  we  are  in  the  rear,  as  shown  in  Fig.  165.  At  A  the  sun 
is  rising :  at  B  it  is  setting.  The  meteors  are  supposed  to  be  com- 
ing from  all  directions. 


SUN 


EARTH1 


Fig.  165. —  RELATIVE  FREQUENCY  OF  METEORS  IN  THE  MORNING  AND  EVENING. 

323.  Paths  and   Velocity.  —  A  shooting  star  which  is  coming  di- 
rectly toward  the  observer  has  no  visible  path.     It  is  simply  an 
evanescent  bright  spot  on  the  sky.     Those  which  shoot  to  one  side 
of  him  usually  have  paths  several  degrees  in  length.     The  paths  of 
some  meteors  exhibit   abrupt  changes  of  curvature :    the  meteors 
appear  to  glance,  as  a  skipping  stone  does  on  the  water. 

By  means  of  observations  taken  by  men  stationed  several  miles 
from  each  other,  shooting  stars  have  been  found  to  be  on  the  aver- 
age seventy-five  miles  above  us  when  they  are  ignited,  and  fifty 
miles  when  they  disappear.  While  glowing  they  travel  forty  or 
fifty  miles  at  rates  of  from  ten  to  fifty  miles  per  second.  Like  the 
meteorites,  they  have  orbits  about  the  sun.  A  few  double  meteors 
have  been  seen  moving  side  by  side,  some  of  them  being  connected 
by  a  ligament  of  light.  They  were  telescopic. 

324.  Masses  and  Constituents.  —  Since   these  bodies  perish  when 
they  have  encountered  the  extremely  rare  atmosphere  which  exists 
at  heights  of  from  fifty  to  seventy-five  miles,  they  must  be  insignifi- 


2l6 


DESCRIPTIVE    ASTRONOMY. 


cant  in  mass.  Most  of  those  which  compose  a  shower  are  believed 
to  be  less  than  a  grain  in  weight.  One  as  brilliant  as  Jupiter  or 
Venus  at  their  best,  may  weigh  50  or  100  grains.  These  rather 
insecure  estimates  are  based  upon  measurements  of  their  light, 
combined  with  those  of  their  velocity.  The  spectroscope  shows 
that  sodium  and  magnesium  are  constituents  of  shooting  stars. 

Some  years  since,  the  Swedish  naturalist  Nordenskiold  melted  a 
quantity  of  snow  taken  from  polar  regions,  and  discovered  in  the 
water  minute  particles  which  proved  to  be  compounds  of  iron  :  they 
were  hailed  as  the  debris  of  shooting  stars. 

But  the  great  eruption  of  Krakatoa  in  1883  taught  us  that  fine 
volcanic  ashes  may  be  carried  in  the  air  for  thousands  of  miles  be- 
fore they  finally  settle  to  the  earth.  The  particles  found  by  Nor- 
denskiold may  therefore  be  the  products  of  volcanic  eruptions. 


Fig.  166.  —  THE  RADIANT. 

325.  Radiant.  —  When  a  shower  of  shooting  stars  comes,  the  ap- 
parent paths  of  the  bodies  when  produced  backward  meet  at  a  spot 
on  the  sky,  which  is  called  the  radiant  point,  or  simply  the  radiant. 
One  who  looks  upward  during  a  gentle  fall  of  snow  will  observe  the 
same  phenomenon  with  reference  to  the  paths  of  the  snowflakes. 

The  paths  prolonged  backward  seem  to  converge  toward  the 
zenith.  But  the  snowflakes  are  descending  in  parallel  paths,  and 
the  convergence  is  explained  as  an  effect  of  perspective.  Hence 


COMETS    AND    METEORS.  217 

we  infer  that  the  shooting  stars  are  coming  in  parallel  paths.  As  the 
hours  of  the  night  wear  away,  the  radiant  remains  in  the  same  place 
among  the  stars  :  from  this  we  conclude  that  the  meteors  come  from 
the  same  direction,  so  that  there  must  be  a  stream  of  them,  pouring 
a  bootless  fusillade  in  upon  the  earth. 

Meteoric  showers  are  frequently  named  from  the  constellation  in 
which  the  radiant  lies.  The  Leonids,  Perseids,  and  Andromedes 
come  from  the  constellations  Leo,  Perseus,  and  Andromeda,  re- 
spectively. 

326.  The   August  Shower.  —  The  August   meteors  are    popularly 
known  as  the  "  Tears  of  St.  Lawrence."     They  are  most  numerous 
about   August   10,  when  an  observer   may  see    one   every  minute, 
the  radiant  being  in  the  constellation  Perseus.     Perseids  are  visible 
in  greater  or  less  numbers  during  the  latter  half  of  July  and  the  first 
three  weeks  of  August  (Fig.  167).     This  shows  that  the  meteoric 
stream  is  very  broad,  for  the   earth  moves  about  60,000000  miles 
while  passing  through  it. 

The  meteors  are  distributed  rather  uniformly  along  the  orbit, 
though  there  are  occasional  gaps.  In  August,  1892,  the  shower  did 
not  come.  The  orbit  is  an  ellipse  extending  beyond  the  orbit  of 
Neptune,  and  the  period  of  revolution  is  over  100  years. 

327.  The  November  Leonids :  Appearance :  Velocity :  Orbit.  —  Every 
year  about  November  13  there  is  a  shower  of  meteors  emanating 
from  the  constellation  Leo.     In  most  years  they  are  rather  insig- 
nificant, but  once  in  33  years  the  magnificence  of  the  display  is 
appalling.     When  the  encounter  takes  place,  the  meteors  come  in 
a  direction  nearly  opposite  to  that  in  which  the  earth  is  moving. 
The  velocity  of  the  meteors  is  26  miles  a  second,  and  that  of  the 
earth   18  miles  a  second,  so  that  the  missiles  pelt  the  earth  as  furi- 
ously as  if  they  were  going  44  miles  a  second,  and  the  earth  were 
at  rest.     They  have  a  brilliant  bluish  light,  and  leave  vivid  trails. 

This  meteoric  system  is  not  diffused  around  its  orbit,  like  the 
August  meteors,  but  is  largely  condensed  in  a  single  shoal  about 
2,000,000000  miles  long.  Their  orbit  is  a  long  ellipse,  the  peri- 
helion of  which  is  near  the  path  of  Uranus.  The  periodic  time  is 
33!  years. 

328.  The  November  Leonids :  Past  Showers.  —  The  earliest  re- 
corded appearance  of  this  shower  was  in  902  A.  D.  On  the  very 


ai8 


DESCRIPTIVE    ASTRONOMY. 


Fig.  167.  —  THE  ORBIT  OF  THE  AUGUST  SHOWER. 
(From  Winchell's  "  World  Life,"  by  permission.) 

night  when  a  Moorish  tyrant  died,  "  by  the  judgment  of  God,"  there 
were  seen,  "  as  it  were  lances,  an  infinite  number  of  stars,  which 
scattered  themselves  like  rain  to  right  and  left."  That  year  was 


COMETS    AND    METEORS.  2IQ 

called  the  "Year  of  the  Stars."  On  the  night  of  Nov.  12,  1833,  the 
display  was  probably  the  most  magnificent  on  record.  The  falling 
stars  were  as  thick  as  flakes  in  a  snowstorm ;  there  were  many  fire- 
balls brighter  than  Venus ;  one  is  said  to  have  looked  larger  than 
the  full  moon.  The  utmost  terror  was  inspired  in  the  ignorant.  A 
South  Carolina  planter  wrote :  — 

"  I  was  suddenly  awakened  by  the  most  distressing  cries  that  ever  fell 
•on  my  ears.  Shrieks  of  horror  and  cries  for  mercy  I  could  hear  from  most 
of  the  negroes  of  the  three  plantations,  amounting  in  all  to  about  six  or 
eight  hundred.  While  earnestly  listening  for  the  cause  I  heard  a  faint 
voice  near  the  door  calling  my  name.  I  arose,  and,  taking  my  sword,  stood 
at  the  door.  At  this  moment  I  heard  the  same  voice  still  beseeching  me 
to  arise,  and  saying,  'The  world  is  on  fire  ! '  I  then  opened  the  door,  and 
it  is  difficult  to  say  which  excited  me  the  most,  —  the  awfulness  of  the 
scene,  or  the  distressed  cries  of  the  negroes.  Upwards  of  one  hundred  lay 
prostrate  on  the  ground,  —  some  speechless,  and  some  with  the  bitterest 
cries,  with  their  hands  raised,  imploring  God  to  save  the  world  and  them. 
The  scene  was  truly  awful ;  for  never  did  rain  fall  much  thicker  than  the 
meteors  fell  to  the  earth  :  east,  west,  north,  and  south,  it  was  the  same." 

In  1866  there  was  another  wonderful  display,  on  the  night  of 
November  13.  The  next  great  shower  is  expected  in  1899;  the 
meteoric  shoal  is  so  long,  that  there  may  be  a  fair  shower  in  1898. 

329.  History  of  the  Leonids.  —  Fig.  168  illustrates  the  supposed 
introduction  of  the  Leonids  into  our  system.  Their  probable  his- 
tory is  as  follows.  Prior  to  the  second  century  of  our  era  they  were 
coming  toward  the  solar  system  in  a  tolerably  compact  swarm  mov- 
ing in  a  parabolic  orbit.  Neptune,  the  first  picket,  was  successfully 
passed.  But  Uranus  came  along  opportunely,  and  in  A.  D.  126  gave 
them  so  powerful  a  tug  that  their  orbit  was  changed  to  the  ellipse 
shown  in  the  figure,  and  they  found  themselves  subject  to  the  sun. 
The  attraction  of  Uranus  also  distorted  the  shoal,  and  caused  its 
various  components  to  move  in  slightly  different  orbits :  the  sepa- 
rate meteors  were  made  to  move  with  slightly  different  velocities,  so 
that  the  shoal  became  elongated.  Time  and  time  again  the  earth 
dashed  through  it.  Furthermore,  the  attractions  of  Jupiter  and 
Saturn  kept  shifting  its  orbit,  until  it  now  occupies  the  position 
shown  by  the  dotted  lines  in  the  figure.  Looking  ahead,  we  may 


220 


DESCRIPTIVE    ASTRONOMY. 


prophesy  that  the  shoal  will,  in  the  course  of  ages,   become  dis- 
tributed around  the  orbit,  as  are  the  August  meteors. 

330.    The  Bielids.  —  This  meteoric  shower  comes  in  the  latter  part 
of  November  each  year.     It  overtakes  the  earth,  striking  it  with  a 


Fig-  168.  —  CAPTURE  OF  THE  LEONIDS. 

relative  velocity  of  only  12  miles  a  second.  The  radiant  point  is  in 
Andromeda.  Brilliant  displays  were  seen  in  1872  and  1885.  In 
1892  the  meteors  were  expected  on  November  27,  but  arrived  on 
November  23.  The  early  arrival  was  afterwards  discovered  to  be 
an  effect  of  Jupiter's  attraction  upon  the  meteoric  stream.  Com- 
parison with  old  records  shows  that  the  shower,  though  somewhat 
irregular  in  the  date  of  its  appearance,  comes  gradually  earlier  and 
earlier,  gaining  a  fortnight  in  a  century.  The  swarm  derives  great 
interest  from  its  supposed  connection  with  Biela's  comet  (§  303). 
The  comet  seems  to  be  lost,  but  a  meteoric  shower  more  pronounced 
than  the  average  comes  when  the  comet  is  due.  Some  writers  speak 
of  these  meteors  as  fragments  of  Biela's  comet.  In  the  shower  of 
1892  several  could  be  seen  every  minute  by  a  single  observer. 
Another  fine  display  is  expected  in  1898. 


COMEtS    AND    METEORS.  221 

331.  Meteoric  and  Cometic  Orbits.  —  As  soon  as  the  orbits  of  the 
great  meteoric  swarms  were  computed,  it  was  perceived  that  they 
were  similar  to  those  of  certain  comets.     The  August  meteor  swarm 
moves  in  an  orbit  which  is  identical  with  that  of  the  bright  comet 
of  1862. 

The  meteors  which  cause  the  great  33  year  November  shower 
follow  hot  on  the  trail  of  Tempel's  comet  (1866,  I.). 
Several  other  similar  coincidences  are  now  known. 

332.  Relation  between  Comets  and  Meteors.  —  The  Bielids  are  con- 
sidered by  many  to  be  portions  of  Biela's  comet. 

The  bright  comet  of  1862  seems  to  be  simply  a  condensed  por- 
tion of  the  August  meteoric  swarm. 

Tempel's  comet  and  the  33  year  shower  may  both  be  parts  of 
the  original  mass  brought  into  our  system  by  the  attraction  of 
Uranus. 

The  general  opinion  is  that  shoals  of  meteoric  matter  ac- 
companying comets  are  the  products  of  the  disintegration  of  the 
cometary  masses. 

333.  How  to  Observe  a  Meteoric  Shower.1  —  In  order  to  calculate 
the  orbit  of  a  shower  it  is  necessary  to  know  the  position  of  the 
radiant  point.     A  careful  map  of  the  stars  visible  to  the  naked  eye 
in  the  vicinity  of  the  radiant  is  first  prepared.     This  may  be  done 
easily  by  putting  a  piece  of  tracing  paper  or  tracing  cloth  over  a 
good  star  map.     On  tracing  cloth  the  stars  may  be  well  marked  by 
small  dots  of  ink.     The  completed  map  should  be  securely  fastened 
to  a  smooth  board.     A  watch  of  the  sky  for  a  few  minutes  during 
the  shower  will  enable  one  to  locate  the  radiant  point  fairly.     With 
eyes  directed  toward  this  spot,  the  observer  notes  the  apparent  path 
of  some  meteor ,'  he  then  traces  it  upon  the  map,  marking  as  accu- 
rately as  possible  the  beginning  and  end  of  the  path.     A  stick  held 
in  the  hand,  and  placed  parallel  to  the  meteor's  path,  will  be  of  de- 
cided assistance.     Those  meteors  having  short  paths  are  best  suited 
for  fixing  the  position  of  the  radiant.     When  the  observations  are 
finished,  the  paths  marked  on  the  map  should  be  prolonged  back- 
ward till  they  intersect :   they  will  not  all  intersect  at  the  same  point. 
After  studying  the  map,  the  observer  should  mark  the  spot  which 

1  See  articles  by  W.  F.  Denning  in  "  Popular  Astronomy  "  for  October,  November, 
and  December,  1893. 


222  DESCRIPTIVE    ASTRONOMY. 

>%*. 

he  deems  to  be  the  true  position  of  the  radiant,  and  find  its  right 
ascension  and  declination. 

Another  person,  or  several  others,  may  make  repeated  counts  of 
the  number  of  meteors  visible  in  ten  minutes,  noting  the  time  when 
each  set  of  observations  was  begun. 

Still  other  observers  may  note  the  paths  of  brilliant  ones,  and  in- 
teresting phenomena  concerning  them,  such  as  their  brightness, 
color  and  length  of  train.  Successful  observations  should  be  pub- 
lished as  recommended  in  the  note  to  §  321. 

The  apparent  paths  of  the  brighter  meteors  can  now  be  de- 
termined with  more  accuracy  by  photography,  than  by  visual 
observations. 

THE   ZODIACAL    LIGHT. 

334.  In  the  early  spring  one  may  see,  when  twilight  has  faded 
away,  a  faint  hazy  beam  of  light  projecting  up  from  the  horizon  in 
the  west.     It  lies  in  the  ecliptic,  and  can  be  traced  90°  or  more  from 
the  sun  without  much  difficulty. 

In  autumn  it  can  be  well  seen  in  the  morning  before  sunrise.  It 
is  said  to  have  been  observed  to  follow  the  ecliptic  clear  around  the 
sky.  There  are  many  theories  as  to  its  cause :  the  most  widely  ac- 
cepted is  that  it  is  due  to  a  countless  host  of  meteoric  bodies  revolv- 
ing about  the  sun,  and  constituting  a  huge  figure  resembling  in 
shape  a  double  convex  lens. 

The  Gegenschein,  or  Zodiacal  Counterglow,  lies  opposite  the 
sun,  and  is  a  very  faint  round  appearance,  a  trifle  brighter  than  the 
adjoining  portions  of  the  zodiacal  light.  Observations  of  it  are 
exceedingly  difficult.1 

EXERCISES. 

335.  I.    May  Comet  a  1907  be  also  Comet  1907  II.? 

2.  Why  are  jets  spurted  out  from  the  nucleus  of  a  comet  on  the 
side  next  to  the  sun  rather  than  on  the  other  side  ? 

3.  Why  are  comets'  tails  when  composed  of  a  heavy  material 
(Type  III.)  more  sharply  curved  than  when  composed  of  a  light 
material  (Type  I.)? 

1  See  Dr.  Barnard's  interesting  and  important  observations,  published  in  No.  308  of 
the  "  Astronomical  Journal." 


COMETS    AND    METEORS.  223 

4.  If  a  comet  were  a  compact  sphere  80  miles  in  diameter,  the 
average  density  of  which  equalled  that  of  the  earth,  its  mass  would 
be  what  fractional  part  of  the  earth's  mass,  the  earth's  diameter  be- 
ing called  8,000  miles  ? 

5.  A  circular  shield  in  front  of  the  earth,  to  protect  it  in  case  of 
collision  with    a  comet,   would   have  an  area  of   about   50,000000 
square  miles  if  it  were  8,000  miles  in  diameter.    Verify  this  statement. 
Suppose  that  1,000,000000  masses  of  the  size  of  a  man's  fist  uni- 
formly distributed  throughout  the  comet  pelted  the  shield  during 
such  a  collision.     Is  it  likely  that  a  particular  house  located  on  the 
front  of  the  shield  would  be  struck? 

6.  If  the  nucleus  of  a  comet  spurts  out  a  jet  toward  the  sun, 
does  that  action  tend  to  drive  the  nucleus  away  from  the  sun? 

7.  If  a  meteorite  overtake  the  earth,  is  it  less  liable  to  be  shattered 
in  the  air  than  if  it  meet  the  earth? 

8.  What  evidence  is  there  that  a  meteorite,  before  it  strikes  the 
atmosphere,  is  a  cold  body? 

9.  On  some  clear  moonless  night  observe  a  meteor,  estimating  its 
time  of  flight,  and  the  direction  in  which  it  moved.     Note  its  color 
and  the  length  of  its  train  (in  degrees),  remembering  that  the  dis- 
tance between  "  The  Pointers  "  in  the  Great  Dipper  is  five  degrees,. 

10.  Do  meteors  gradually  increase  the  size  of  the  eajth? 

11.  Do  those  meteors  which  meet  the  earth,  and   thus  resist  its 
motion,  tend  to  lengthen  or  shorten  the  year?     (§  302.) 

12.  The  sun  must  be  struck  by  meteors,  as  well  as  the  earth. 
Do  meteors    tend  to  increase  the  attraction  between  the    sun  and 
the  earth? 

13.  If  the  attraction  between  the  sun  and  the  earth  be  thus  in- 
creased, does  the  increase  operate  to  shorten  the  year? 

14.  Would  a  decrease  of  the  distance  between  the  sun  and  the 
earth  operate  to  shorten  the  year? 

15.  Judging  by  the  effect  of  meteors  on  the  temperature  of  the 
earth,  do  you  think  that  the  sun's  heat  can  be  accounted  for  by 
their  impact? 


224 


DESCRIPTIVE    ASTRONOMY. 


Fig.  169.  — STARS  VISIBLE  TO  THE  NAKED  EYE. 


THE    FIXED    STARS.  225 


CHAPTER   XII. 

THE    FIXED    STARS. 

"  What  involution  !  what  extent !   what  swarms 
Of  worlds,  that  laugh  at  earth  !   immensely  great ! 
Immensely  distant  from  each  other's  spheres ! " 

YOUNG. 

336.  Number  Visible. —  The  stars  visible  to  the  naked  eye  are  by  no 
means  countless. ,.  Over  2,000  of  them  can  be  seen  at  one  time,  under 
favorable  circumstances.     Were  it  possible  to  see  the  entire  celestial 
sphere  as  well  as  one  sees  the  portion  of  it  near  the  zenith,  more  than 
6,000  stars  could  be  enumerated.     There  are  multitudes  of  stars  just 
below  the  limit  of.  naked-eye  vision  which  a  spy-glass  brings  out 
without  difficulty  (Fig.  169).     It  will  show  twenty  stars  for  every  one 
seen  without  its  aid.     The  forty-inch  Yerkes  telescope  (Fig.  170)  of 
the  University  of  Chicago  may  be  capable  of  revealing  1,000  times 
as  many  stars  as  a  lady's  opera-glass.     As  faint  stars  are  much  more 
numerous  than  bright  ones,  only  a  few  hundred  stars  can  be  seen 
without  a  telescope  when  the  full  moon  dominates  the  heavens. 

337.  Scintillation.  —  Comparison    of   stars   near   the    zenith    with 
those  at  a  greater  distance  from  it  shows  that  stars  twinkle  the  more, 
the  nearer  they  are  to  the  horizon.     Since  the  light  coming  from 
those  near  the  horizon  passes  through  a  greater  thickness  of  air  than 
that  from  those  at  higher  altitudes,  it  is  more  violently  disturbed. 
The  strata  of  air  through  which   it  passes   have  many  degrees  of 
density,  so  that  the  light  is  refracted  hither  and  thither  on  its  way  to 
the  eye.     The  image   of  a  star  in  a  telescope  is  made   to  boil  or 
dance.     The  irregular  refraction  of  the  light  also  causes  a  phenome- 
non described  in  works  on  physics  as  u  interference."      In  conse- 
quence of  this  there  are  continual  changes  of  color;   the  flashes  of 
many  colors  which  emanate  from  Sirius,  the  brightest  of  the  fixed 
stars,  when  it  is  near  the  horizon  in  the  early  evening  in  midwinter, 
are  very  beautiful.     An    electric    arc   light   scintillates    when    seen 
at  a   distance    of  several    blocks.     Scintillation    is    generally    most 
marked  on  windy  or  frosty  nights.     When  the   stars    twinkle    vio- 


226 


DESCRIPTIVE    ASTRONOMY. 


lently,  telescopic  observations  are   usually  of  little  value.     A  little 
haze  uniformly  suffused  through  the  atmosphere  reduces  scintillation 


Fig.  170.  —  THE  YERKES  TELESCOPE  AT  THE  WORLD'S  FAIR,  1893. 

to  a  minimum :   on  a  slightly  hazy  night  the  stellar  images  seen  in  a 
telescope  are  in  general  small  and  neat,  and  bear  magnifying  well. 


THE    FIXED    STARS.  22  J 

The  planets,  having  much  broader  disks  than  the  fixed  stars, 
twinkle  very  little ;  when  one  point  of  their  disks  is  temporarily  dark- 
ened by  interference,  another  may  become  brighter,  so  that  the  total 
quantity  of  light  from  all  points  of  the  disk  remains  about  the  same. 


Fig.  171.  —  A  PORTION  OF  THE  MILKY  WAY. 

338.   The  Milky  Way.  — The  Milky  Way,  or  Galaxy,  is  the  broad 
bright  stream  of  stars  which  encircles   the  heavens,  and  exhibits  a 


228  DESCRIPTIVE    ASTRONOMY. 

fine  contrast  with  the  blackness  of  the  sky  at  times  when  the  moon 
is  not  visible.  The  Galaxy  is  not  of  uniform  brightness ;  in  places 
there  are  striking  dark  spots,  many  of  which  look  like  vast  abysses : 
one  in  the  constellation  of  the  Centaur  is  called  the  Coal  Sack :  in 
Cygnus  there  is  another  starless  space,  smaller  than  the  Coal  Sack. 
The  portion  of  the  Milky  Way  seen  south  of  the  zenith  in  middle 
north  latitudes  in  midsummer  is  divided  into  two  streams,  lying  side 
by  side.  The  sheen  of  the  Galaxy  is  due  to  the  fact  that  it  is  com- 
posed of  millions  of  closely  packed  faint  stars.  In  the  brighter  por- 
tions of  it,  hundreds  of  stars  can  be  seen  in  the  field  of  view  of  an 
opera-glass.  Photographs  taken  with  large  lenses  and  long  expos- 
ures show  that  there  is  a  marvellous  complexity  of  structure ;  there 
are  sprays  of  stars,  and  vast  cloud-like  appearances,1  which  are 
crossed  by  dark  lanes  and  bestrewn  with  dark  spots. 

339.  Tree-like  Structures.  - —  Many  solar  prominences  have  tree-like 
forms :   one  is  astonished  to  find  such  forms  in  the  Milky  Way,  but 

photography  gives  indubitable  evidence  of  their  exist- 
ence. Some  of  these  are  dark,  and  others  are  bright. 
Fig.  172  represents  a  dark  plant-like  structure  near 
Alpha  Cygni,  which  appears  on  a  photograph  taken 
by  Dr.  Wolf2  with  an  exposure  time  of  eleven  hours. 
It  has  been  conjectured  that  these  forms  are  due  to 
colossal  uprushes  into  a  resisting  medium.  But  the 
dimensions  of  these  mysterious  objects  are  so  enor- 
mous that  this  explanation  seems  inadequate.  The 
matter  is  still  further  complicated  by  the  fact  that 
Fig.  172.  — PLANT-  manv  of  the  dark  structures  are  bordered  by  lines  of 

LIKE  STRUCTURE. 

stars. 

340.  The   Constellations.  —  In   the    early   ages  men    grouped    the 
brighter  stars  fantastically,  and   gave  names  to  the  groups.      The 
Latin  forms  of  these  names  are  now  employed.     Some  of  the  most 
commonly   known  constellations  visible  in   the    United    States   are 
Cassiopeia  (the  Lady  in  the  Chair),  Cygnus  (the  Swan),  Leo  (the 
Lion),  Lyra  (the  Lyre),  Orion,  and  Ursa  Major  (the  Great  Bear). 
Most  of  the  constellations  are  named  after  mythological  personages, 
or  after  animals.     Ptolemy,  who  died  A.  D.  170,  revised  the  scheme 

1  Dr.  Barnard  first  photographed  these. 

2  Dr.  Max  Wolf,  of  Heidelberg,  one  of  the  foremost  of  celestial  photographers. 


THE    FIXED    STARS.  22Q 

of  constellations  known  to  the  ancients  and  transmitted  to  us  forty- 
eight  of  them.  More  modern  astronomers  have  added  a  large  num- 
ber of  other  constellations  to  fill  in  the  spaces  not  covered  by  the  old 
ones.  Nineteen  of  these  are  now  generally  recognized. 

341.  Names  of  Individual  Stars.  —  Many  of  the  brighter  stars  have 
received  proper  names,   drawn  from  the  Latin,  Greek,  and  Arabic 
languages.     Such  are  Sirius,  Polaris,  Rigel,  Aldebaran,  and  Vega. 
Practical   astronomers   use    only   a   few    of  them.     Such  names  as 
Skat,  Rotanev,   Muphrid,   and    Zavijava  are  sinking   into  deserved 
oblivion. 

Naked-eye  stars  are  most  commonly  designated  by  letters  or 
numbers  prefixed  to  the  genitive  case  of  the  Latin  name  of  a  con- 
stellation. The  general  plan  is  to  call  the  brightest  star  in  a  given 
constellation  by  the  Greek  letter  Alpha,  the  next  brightest  being 
Beta,  and  so  on  through  the  alphabet.  Thus  Alpha  Lyrae  is  the 
brightest  star  in  Lyra.  After  the  Greek  alphabet  (§  405)  is  ex- 
hausted, the  Roman  is  used.  Thus  we  have  such  names  as  Delta 
Herculis  and/"Herculis.  The  system  of  Greek  and  Roman  letters 
does  not  follow  the  order  of  brightness  accurately.  There  is  a  sys- 
tem of  numbering  which  is  independent  of  the  other  two,  the  stars 
of  a  constellation  being  numbered  according  to  the  order  in  which 
they  cross  the  meridian.  For  instance  I  Orionis  crosses  before 
2  Orionis.  A  star  may  have  both  a  letter  and  a  number,  the  former 
being  preferred :  Eta  Aurigae  is  the  same  star  as  10  Aurigae. 

For  telescopic  stars  the  names  are  taken  from  catalogues. 
Lalande  19,486  is  the  1 9,486th  star  in  Lalande's  catalogue  of 
stars  -(§  344). 

342.  Orders  of  Brightness.  —  Stars  visible  without  telescopic  aid  are 
divided  into  six  orders  of  brightness,  called  magnitudes  (§  i).     Stars 
of  the  sixth  magnitude  are  just  perceptible  by  an  ordinary  eye :   two 
thousand  of  them  are  visible  in  the  United  States.     A  few  of  them 
may  be  seen  within  the  bowl  of  the   Great  Dipper.     Those  of  the 
fifth  magnitude  are  plain  to  persons  who  are  not  short-sighted.     The 
uppermost  of  the  three  distinct  stars  in  the  sword-handle  of  Orion 
is  of  the  fifth  magnitude.     There  are  twenty  stars  of  the  first  magni- 
tude, fourteen  of  which  lie  between  the  north  pole  and  35°  of  south 
declination  (§  122),  and  can  be  seen  from  any  point  in  the  United 
States.     Polaris  is  of  the  second  magnitude.     An  average  star  of 


230  DESCRIPTIVE    ASTRONOMY. 

the  first  magnitude   is   one   hundred   times  as  bright  as  one  of  the 
sixth  magnitude. 

343.  Magnitudes  of  Telescopic  Stars :  Ratio  of  Magnitude.  —  The  sys- 
tem of  magnitudes  outlined  in  the  preceding  section  is  extended  to 
telescopic  stars.     Of  late  years  a  uniform  ratio  of  brightness  between 
stars  of  successive  magnitudes  has  been  adopted  by  astronomers ;   it 
is  called  the  "light  ratio."     A  star  of  a  given  magnitude  is  'V/ioo 
times  as  bright  as  a  star  of  the  next  lower  magnitude:    -v/ioo  =  2.5 
nearly.     A  star  of  the  third  magnitude  is  2.5  times  as  bright  as  one 
of  the  fourth.     The  forty-inch  telescope  mentioned  in  §  336  should 
reveal  stars  of  the  seventeenth  magnitude.     Stars  even  fainter  than 
this  can  be  photographed.     The  brightness  of  a  star  the  magnitude 
of  which  is  between  two  integral  magnitudes  is  expressed  by  the  aid 
of  decimals.     Thus  a  star  of  the  6.4  magnitude  is  fainter  than  one  of 
the  sixth,  and  brighter  than  one  of  the  seventh  magnitude. 

344.  Star  Catalogues.  —  An  observer  with  a'  meridian  circle  and  a 
clock  can  determine  the  right  ascensions  and  declinations  of  a  large 
number  of  stars,  as  explained  in  Chapter  VI.     These,  when  arranged 
in  the  order  of  their  right  ascensions,  constitute  a  star  catalogue. 
The  names  of  the  stars   and   their  magnitudes  are  also  given.     A 
small  catalogue  is  given  each  year  in  the  Nautical  Almanac.     Each 
of  the  star  places  given  in  this  catalogue  depends  upon  hundreds 
of  observations. 

345.  Photographic  Star  Charts.  —  Stars  whose  right  ascension  and 
declination  have  been  found  can  be  charted,  but  the  work  is  very 
laborious.     Charts  are  made  much  more  expeditiously  by  the  use 
of   photographic    plates.      A    number   of    observatories,    scattered 
throughout  the  world,  are  now  (1896)  engaged  in  securing  photo- 
graphs of  the  entire  sky ;   the  plates  will  exhibit  millions  of  stars 
which  have  never  been  catalogued. 

Prof.  E.  C.  Pickering  has  planned  a  similar  campaign  with  the 
Bruce  photographic  telescope.  This  instrument  should  do  such 
work  with  much  greater  rapidity  than  others  hitherto  constructed. 
Its  objective  is  a  quadruple  lens,  two  feet  in  aperture. 

346.  Distribution.  —  The  stars  visible  to  the  naked  eye  are  distrib- 
uted over  the   face  of  the  sky  with  tolerable  uniformity.     This  is 
shown  in  Fig.  169.     When  telescopic  stars  are  taken  into  consider- 
ation the  case  is  very  different.     These  are  massed  in  great  profu- 


THE    FIXED    STARS.  23! 

sion  in  and  near  the  Milky  Way.  The  farther  one  goes  from  the 
Galaxy,  the  fewer  the  stars  become.  This  fact  was  established  by 
the  observations  of  the  Herschels,  father  and  son,  who  pointed  a 
large  telescope  equipped  with  a  certain  magnifying  power  to  a  few 
thousand  different  places  in  the  sky,  and  counted  the  number  of 
stars  visible  in  the  field  of  view  each  time.  In  the  Galaxy,  122  stars, 
on  the  average,  were  visible  at  one  time  in  the  field  of  view;  15° 
from  the  Galaxy,  only  thirty  were  similarly  brought  into  view;  at 
a  distance  of  30°,  only  eighteen  were  seen;  at  45°,  ten  were 
counted  ;  90°  away,  the  average  number  sank  to  four.  There  is  here 
a  resemblance  to  the  distribution  of  vegetation  on  the  earth  :  it  is 
most  luxuriant  at  the  equator,  and  diminishes  as  one  goes  toward 
the  poles. 

347.  Clusters.  —  The  unassisted  eye  reveals  several  coarse  clusters, 
of  which  the  best  known  is  the  Pleiades,  in  the  constellation  Taurus. 
The  Hyades  in  the  same  constellation,  Praesepe  in  Cancer,  and  the 
cluster  in  the  sword-handle  of  Perseus  are  plain  to  the  naked  eye, 
and  will  well  reward  the  trouble  of  looking  at  them  with  an  opera- 
glass.  In  some  telescopic  clusters,  the  stars  crowd  upon  one 
another  so  closely  that  the  telescope  cannot  separate  them  ;  near 
the  centre  of  the  well  known  cluster  in  Hercules  star  crowds  upon 
star  in  a  blaze  of  glory  (Fig.  173).  When  looking  at  one  of  the 
closely  packed  clusters  with  a  large  telescope,  one  is  apt  to  get  the 
impression  that  he  is  gazing  across  measureless  vistas  of  space  at  a 
remote  system  of  stars,  which  appear  faint  and  crowded  together 
on  account  of  their  stupendous  distance.  But  such  is  not  the  case. 
The  best  evidence  at  command  renders  it  practically  certain  that 
their  distances  from  us  are  no  greater  than  those  of  more  scattered 
stars.  Though  the  stars  in  a  given  cluster  are  supposed  to  be  near 
enough  to  one  another  to  be  subject  to  considerable  mutual  attrac- 
tion, no  motion  due  to  such  a  force  has  yet  been  detected.  Their 
motions  will  probably  be  recorded  on  the  photographic  plates  in 
due  time.  ^^^ 

Ranyard  1  held  that  there  is  evid^fcfc||        c°Uisi°ns  are  taking 


place  between  the  stars,  as  a  result  of  jKir  mutual  attraction. 
If  two  such  bodies  collided,  the^rapid^TOlution  of  heat  at  their 
point  of  contact  would  expand  t!?!*^gtiguous  gases  so  violently 

1  The  late  Mr.  A.  C.  Ranyard,  who  was  editor  of  "Knowledge,"  London,  Eng. 


232  DESCRIPTIVE    ASTRONOMY. 

that  the  effect  of  an  explosion  would  be  produced,  and  the  stars 
might  rebound  like  caroming  billiard  balls,  while  the  gases  heated 
at  the  point  of  impact  would  diffuse  themselves  in  the  surrounding 
space.  To  this  he  attributed  the  radiated  appearance  of  the  outlying 
stars,  which  are  frequently  arranged  in  streams,  as  if  ejected  from 
the  central  mass.  The  stars  in  a  stream  are  often  connected  by  a 
band  of  nebulous  matter.  Photography  has  revealed  the  fact  that 
nebulosity  is  associated  with  very  many  star  clusters. 


Fig.  173.  —  THE  GREAT  CLUSTER  IN  HERCULES. 

348,  Dimensions  and  Nature  of  the  Stars.  —  The  larger  the  magnify- 
ing power  employed,  the  larger  does  a  near  object,  like  the  moon  or 
a  major  planet,  appear.  The  diameters  of  these  bodies  have  been 
measured.  But  with  the  fixed  stars  the  case  is  entirely  different. 
The  larger  and  more  perfect  the  telescope,  the  smaller  the  disk  of  a 
star,  under  the  best  atmospheric  conditions.  The  visible  disk  is  a 
spurious  one,  the  cause  of  which  is  explained  in  works  on  optics. 


THE    FIXED    STARS. 


233 


The  diameter  of  the  sun,  as  seen  from  the  nearest  fixed  star,  is 
equivalent  to  that  of  a  small  marble  a  thousand  miles  from  the  ob- 
server. Though  the  diameters  of  stars  subtend  so  small  a  visual 
angle  as  to  defy  direct  measurement,  yet  the  spectroscope  has  given 
us  a  little  knowledge  concerning  them.  The  star  Algol  (§  379) 
has  been  shown  to  be  over  a  million  miles  in  diameter :  it  has  an 


Fig.  174.  —  THE  CLUSTER  OMEGA  CENTAURI  :   PHOTOGRAPHED  BY  DR.  GILL, 
AT  THE  CAPE  OF  GOOD  HOPE. 

invisible  companion  of  nearly  the  same  size  as  the  sun.  Arcturus, 
Capella,  and  Vega  are  believed  to  be  much  larger  than  our  sun : 
the  diameter  of  Arcturus  may  be  a  hundred  times  that  of  the  sun. 
The  second  magnitude  star  in  the  crook  of  the  handle  of  the  Great 
Dipper  (Zeta  Ursae  Majoris)  is  forty  times  as  massive  as  the  sun. 
The  spectroscope  has  also  shown  that  the  stars  are  self-luminous 
bodies,  similar  to  the  sun. 


234  DESCRIPTIVE    ASTRONOMY. 

349.  Distances.  —  The  distances  of  the  stars  are  inconceivably 
great,  though  easily  expressed  in  figures.  Alpha  Centauri  is  nearer 
than  any  other  star  the  distance  of  which  has  yet  been  measured. 
Its  distance  from  us  is  275,000  times  the  distance  of  the  earth  from 
the  sun ;  light  consumes  over  four  years  in  traversing  this  distance. 
Sirius,  the  Dog  Star,  is  twice  as  far  away.  Most  of  the  stars  are  at 
distances  so  stupendous  as  to  defy  measurement.  It  is  considered 
probable  that  the  vast  majority  of  the  stars  are  so  far  away  that  their 
light  occupies  over  a  century  in  coming  to  us.  The  light  which 
reaches  our  eyes  from  many  a  one  of  them,  may  have  been  on  the 
journey  for  thousands  of  years.  Less  than  one  hundred  stars  have 
been  found  to  lie  within  measurable  distances. 

In  estimating  such  stupendous  distances  it  is  convenient  to  em- 
ploy as  a  unit  the  "  light-year,"  that  is,  the  distance  over  which  light 
would  travel  in  a  year.  A  locomotive  with  driving-wheels  60  miles 
in  diameter,  which  were  revolving  1,000  times  a  second,  would  trav- 
erse the  distance  in  a  few  days  less  than  a  year. 

EARTH  STAR 


Fig.  175.  —  STELLAR  PARALLAX. 

350.  Stellar  Parallax.  —  The  annual  parallax  of  a  star  is  the 
apparent  semidiameter  of  the  earth's  orbit  as  seen  from  the  star. 
This  angle,  formed  at  the  star  by  the  two  lines  shown  in  Fig.  175,  is 
very  small.  In  the  case  of  Alpha  Centauri,  our  nearest  neighbor, 
the  parallax  is  only  as  great  as  the  apparent  diameter  of  a  sphere 
one  foot  through,  located  fifty  miles  from  the  observer. 

FAINT    . 
STARS 


Fig.  176.  —  METHOD  OF  OBSERVING  STELLAR  PARALLAX. 

The  method  most  employed  for  determining  parallax  is  illustrated 
in  Fig.    176.      Usually   some  stars   much  fainter  than  the  star  the 


THE    FIXED    STARS. 


235 


parallax  of  which  is  sought  can  be  seen  in  the  same  telescopic  field 
of  view  with  it.  It  is  reasonable  to  suppose  that  these  are  much 
further  away.  When  the  earth  is  at  E  the  star  A  will  appear  to  be 
situated  at  the  point  X  among  the  fainter  stars.  Six  months  later, 
when  the  earth  is  at  E',  186,000000  miles  from  its  former  position, 
the  star  A  will  appear  to  be  located  at  Y.  This  apparent  shift  of 
position,  when  accurately  measured,  gives  an  astronomer  the  means 
of  computing  the  star's  parallax.  The  explanation  of  the  methods 
of  observation  and  calculation  by  which  the  parallax  is  found  lies 
beyond  the  scope  of  this  book.  The  parallax  being  known,  it  is  a 
simple  matter  to  find  out  how  far  away  the  star  is.  For  the  semi- 
diameter  of  the  earth's  orbit  is  93,000000  miles,  and  the  parallax  is 
the  angle  at  the  star  in  Fig.  175.  The  problem  then  is  to  find 
how  far  from  the  star  a  line  93,000000  miles  long  must  be,  in  order 
to  subtend  a  visual  angle  equal  to  the  star's  parallax.  Its  solution  is 
given  in  the  next  section. 

\ 


Fig.  177.  —  RELATION  OF  PARALLAX  TO  DISTANCE. 

351.  Solution  of  the  Problem.  —  In  Fig.  177  let  the  star  be  at  A, 
the  sun  at  S,  and  the  earth  at  E.  Then  the  angle  SAE  is  the 
star's  parallax ;  this  angle  is  so  minute  that  the  arc  SE  is  practically 
of  the  same  length  as  its  chord,  and  we  may  use  d  to  designate 
either  of  them.  Represent  the  distance  AS  by  R,  and  let  p  equal 
the  number  of  seconds  in  SAE.  The  circumference  of  the  circle  of 
which  SE  is  an  arc  is  2  TT  R.  There  are  360°,  or  1,296000",  in  the 
circumference;  hence  the  length  of  an  arc  of  i"  is  l  229g^00  >  which 
equals  2 0 52 e 5  *  ^e  length  of  an  arc  of  p  seconds  equals  p  times 
this  expression. 


Hence  <*= 


whence     R= 


236  DESCRIPTIVE    ASTRONOMY. 

According  to  this  formula,  if  any  star  had  a  parallax  as  large  as 
five  seconds,  its  distance  from  the  sun  would  be  2  ° 652 6  5 ,  or  41, 253 
times  the  earth's  distance  from  the  sun. 

352.  Colors.  —  Most  of  the  stars  are  white :  there  are  many  of  a 
yellowish  or  reddish  tinge.  A  few  are  of  very  pronounced  colors : 
Sirius  is  white ;  Vega  has  a  bluish  tinge ;  Arcturus  is  reddish.  A 
few  faint  stars  are  deep  red.  Those  which  are  close  to  brighter  ones 
are  usually  bluish  or  greenish. 

It  was  once  thought  that  the  color  gave  a  clue  to  the  temperature 
of  a  star,  the  white  stars  being  much  hotter  than  the  red  ones,  just 
as  white-hot  iron  is  at  a  higher  temperature  than  red-hot.  But  this 
theory  is  now  abandoned :  the  colors  are  probably  dependent  on 
the  materials  which  enter  into  the  composition  of  the  star,  as  well 
as  its  temperature.  The  references  to  the  color  of  Sirius  by  ancient 
writers  render  it  highly  probable  that  it  was  red  at  the  beginning  of 
the  Christian  era. 

353.  Spectra.  —  Secchi,  an  Italian  astronomer,  divided  stellar 
spectra  into  four  arbitrary  classes,  or  types.1 

Type  I.  The  dark  lines  due  to  hydrogen  are  very  pronounced ; 
other  lines  are  few  and  inconspicuous.  This  type  embraces  the 
majority  of  the  stars ;  their  colors  are  white  or  bluish.  They  are 
called  Sirian  stars,  as  Sirius  belongs  to  the  group. 

Type  II.  The  spectrum  resembles  that  of  the  sun,  being  crossed 
by  numerous  dark  lines,  indicating  the  presence  of  various  metals. 
These  are  called  solar  stars,  and  their  colors  are  mostly  yellow. 
Our  nearest  neighbors  among  the  stars  have  recently  been  shown  to 
belong  to  this  type. 

Type  III.  In  spectra  of  this  type,  shaded  bands  are  seen,  each 
of  which  is  darkest  at  the  edge  nearest  the  violet  end  of  the 
spectrum,  and  shades  off  toward  the  red  end.  The  color  of  these 
stars  is  orange  or  red. 

Type  IV.  As  in  Type  III.  we  have  here  a  banded  spectrum,  but 
the  bands  are  darkest  at  the  edge  nearest  the  red  end  of  the  spec- 
trum, and  shade  off  toward  the  violet  end.  These  stars  are  faint,  red, 
and  few  in  number. 

To  these  a  fifth  class  is  now  added,  embracing  the  so  called 
"  Wolf-Rayet "  stars,  which  have  bright  line  spectra.  More  than 
fifty  of  these  are  known. 

1  Some  of  these  spectra  are  shown  in  the  frontispiece. 


THE    FIXED    STARS.  237 

354.  Discussion  of  Stars  of  Different  Spectral  Types.  —  Two   thirds 
of  the  Sirian  stars  are  situated  in  the  Milky  Way,  while  the  solar 
stars  are  about  evenly  divided  between  galactic  and  non-galactic 
regions.     Each    square    mile    of  the    surface    of  a    Sirian    star    is 
brighter  than  an  equal  area  of  a  solar  star,  but  solar  stars  are  on 
the  average  much  more  massive  than   Sirians,   and    give  a  greater 
quantity  of  light. 

The  Wolf-Rayet  or  "  bright-line  "  stars  lie  in  or  near  the  Milky 
Way :  these  stars  are  of  special  interest,  because  they  apparently 
form  a  connecting  link  between  the  nebulae  (§  388)  and  other  stars. 
Bright  lines  are  not  uncommonly  found  in  the  spectrum  of  the  sun 
itself,  and  are  thought  to  be  due  to  masses  of  vapor  hotter  than  the 
underlying  photosphere. 

The  stars  in  Orion  (with  the  notable  exception  of  Betelgeuse) 
have  a  special  variety  of  spectrum,  scarcely  found  outside  of  that 
constellation.  This  indicates  that  these  stars  have  a  similar  structure ; 
probably  they  are  "  chips  off  the  same  block." 

"  In  general,  it  may  be  stated  that,  with  a  few  exceptions,  all  the 
stars  may  be  arranged  in  a  sequence,  beginning  with  the  planetary 
nebulae  (§  385),  passing  through  the  bright-line  stars  to  the  Orion 
stars,  thence  to  the  first  type  stars,  and  by  insensible  changes  to 
the  second  and  third  type  stars.  The  evidence  that  the  same  plan 
governs  the  constitution  of  all  parts  of  the  visible  universe  is  thus 
conclusive."  l 

Different  spectra  doubtless  indicate,  in  many  cases,  different  stages 
of  evolution,  but  many  more  observations  must  be  made  before  any 
far  reaching  theory  can  be  suitably  fortified. 

355.  Light  and  Heat.  — The  amount  of  light  received  from  some  of 
the  stars  has  been  compared  with  that  given  us  by  the  sun.    Though 
Sirius  far  outshines  any  other  fixed  star,  being  nearly  six  times  as 
bright  as  Vega,  7,000,000000  stars  like  it  would   be  required  to  fur- 
nish daylight ;   9,000000,000000  stars  of  the  sixth  magnitude  would 
be  necessary  for  the  same  purpose. 

Professor  Young  estimates  that  the  full  moon  gives  sixty  times  as 
much  light  as  the  entire  starry  sphere ;  and  that  ninety-five  per  cent 
of  the  latter  comes  from  stars  invisible  to  the  naked  eye. 

1  Prof.  E.  C.  Pickering,  in  "  Astronomy  and  Astrophysics"  for  October,  1893. 


238  DESCRIPTIVE    ASTRONOMY. 

No  trustworthy  measures  of  the  heat  reaching  the  earth  from  any 
particular  star  have  been  made.  It  is  too  small  to  affect  the  most 
delicate  thermometric  appliances. 

356.  Bird's-eye  View  of  the  Stellar  System.  —  The  following  conclu- 
sions have  been  reached  by  a  study  of  the  star  gauges  made  by  the 
Herschels,  assuming  that  the  faint  stars  are,  as  a  class,  more  distant 
than  the  bright  ones.     Though  subject  to  considerable  uncertainty, 
they  are  generally  given  in  works  on  descriptive  astronomy. 

(a)  Most  of  the  stars  are  not  arranged  in  the  form  of  a  sphere, 
but  in  that  of  a  thin  disk ;   the  shape  of  the  disk  is  about  that  of  a, 
silver  dollar. 

(b)  Only  a  small  proportion  of  the  stars  lie  on  one  side  or  the 
other  of  the  disk,  but  the  majority  of  the  nebulae  find  their  homes 
there ;   i.  e.  outside  of  the  disk. 

(c)  Within  the  disk  are  the  stars  embraced  in  the  Milky  Way,, 
which  contains  most  of  the  Sirian  stars. 

(d)  The   stars  are   not   evenly    distributed  throughout  the  disk. 
The  fainter  ones  are  grouped  in  clusters  and  streams,  as  a  nation  is. 
divided  into  families.     Many  of  the  brighter  stars  are  thus  grouped, 
but  each  "  family"  consists  of  fewer  individuals  than  in  the  case  of 
the  faint  ones. 

(e)  The  sun  lies  near  the  centre  of  the  disk. 

357.  Kapteyn's  Investigations.  —  Prof.  J.  C.  Kapteyn,  a   Dutch   as- 
tronomer, has  made  the  most  exhaustive  discussion  of  the  form  of 
the  sidereal  universe.     The  most  interesting  of  his  conclusions  may 
be  summed  up  under  three  heads. 

(a)  The  nearer  stars  are  chiefly  of  the  solar  spectral  class,  and  are 
scattered  about  the  sun  on  all  sides,  independently  of  the  position  of 
the  Milky  Way.  They  form  with  the  sun  a  scattered  cluster. 

(#)  Those  stars  the  distances  of  which  from  us  are  immeasurably 
great,  whether  Sirian  of  solar,  are  more  numerous  the  nearer  they 
lie  to  the  plane  of  the  Milky  Way. 

(c)  Of  the  stars  of  any  given  brightness  (say  sixth  magnitude),, 
those  which  lie  in  or  near  the  Milky  Way  are,  on  the  whole,  more 
remote  from  us  than  those  which  lie  in  other  parts  of  the  heavens. 

The  stellar  universe  thus  bears  a  rude  resemblance  to  the  planet 
Saturn,  consisting  of  a  central  ball  of  stars,  surrounded  at  a  great 
distance  by  an  apparently  ring-shaped  collection  of  stars.  Professor- 


THE    FIXED    STARS.  239 

Kapteyn  likens  it  to  the  nebula  in  Andromeda  (§  389),  the  cen- 
tral nucleus  of  which  corresponds  to  the  solar  cluster,  while  the  out- 
lying whorls  are  miniatures  of  the  Galaxy. 

358.  Proper  Motions.  —  Though  the  stars  are  called  "  fixed,"  they 
are  far  from  being  so.     They  are  moving  with  various  degrees  of 
rapidity  in  all  conceivable  directions ;   but  on  account  of  their  pro- 
digious   distances  from  the  earth,  their  positions  with  reference  to 
one   another  change  by  minute   amounts  only,  from  year  to  year. 
Proper  motion  is  this  apparent  shifting  of  a  star's  position  on  the 
celestial  sphere. 

The  proper  motion  is  not  the  star's  real  motion  in  space.  If  the 
earth  were  at  rest,  and  a  star  were  coming  directly  toward  it  or  going 
directly  away  from  it,  the  star  would  appear  to  be  fixed  on  the 
celestial  sphere,  and  would  have  no  proper  motion.  The  largest 
proper  motion  yet  discovered  is  that  of  the  star  Groombridge  1830, 
which  has  been  graphically  termed  the  "  runaway  star."  In  270 
years  it  moves  over  a  space  equal  to  the  apparent  diameter  of  the 
moon.  The  bright  stars  have,  on  the  average,  larger  proper  motion 
than  the  faint  ones.  This  is  probably  due  to  the  fact  that  their 
average  distance  from  us  is  less  than  that  of  the  faint  stars.  Arc- 
turus  has  apparently  moved  over  a  space  equal  to  one  fifth  of  the 
distance  between  the  Pointers  in  the  Great  Bear,  during  the  Chris- 
tian era. 

It  has  been  shown,  by  combining  a  mass  of  observations  on  numer- 
ous stars,  that  the  average  proper  motion  of  a  first  magnitude  star  is 
six  times  that  of  one  of  the  sixth  magnitude.  Stars  having  a  large 
proper  motion  are  at  less  distances  from  us,  on  the  whole,  than 
those  of  small  proper  motion. 

359.  Proper  Motion  Groups.  —  While  proper  motions  have  all  sorts 
of  directions,  there    are  many  groups  of  stars  the  components  of 
which  move  in  the  same  direction.     The  stars  in  the  Great  Dipper, 
with  the  exception  of  the  one  at  each  end  of  the  figure,  belong  to 
such  a  group.     By  spectroscopic  observations  (§  360)  it  has  been 
found  that  these  five  stars  are  all  retreating  from  us  to  greater  depths 
of  interstellar  space.     They  are    separated    from    one    another   by 
distances  inconceivably  vast.     Yet  there  seems  to  be  some  common 
bond,  and  the  laws  of  motion  of  this  stupendous  system  may  for- 
ever elude  the  keenest  search  of  man. 


240 


DESCRIPTIVE    ASTRONOMY. 


f 

I 


Fig.  178.  —  PROPER  MOTIONS  OF  THE 
PLEIADES. 


The  Pleiades  constitute  a  similar  system.  Of  the  four  hundred 
stars  catalogued  in  this  cluster,  only  a  few  refuse  to  conform  to  a 

common  proper  motion  pos- 
sessed by  the  others.  These 
outre  stars  are  probably  be- 
tween us  and  the  cluster 
proper,  or  beyond  it.  The 
other  stars  all  agree  in  hav- 
ing the  same  spectra. 

360.  Motions  in  the  Line  of 
Sight.  —  Every  star  is  con- 
stantly sending  forth  lumi- 
nous vibrations  of  various 
wave  lengths.  If  the  star  be 
approaching,  the  number  of 
waves  which  reach  us  in  any 
given  time  is  increased,  and 
their  wave  length  is  shortened.  Hence,  when  a  star  is  coming  to- 
ward us,  its  light  is  rendered  more  refrangible,  and  all  of  the  lines 
in  its  spectrum  are  shifted  toward  the  violet  end  of  the  spectrum. 
By  comparing  the  position  of  the  hydrogen  lines,  for  example,  in 
the  spectrum  of  the  star,  with  the  spectrum  of  hydrogen  obtained 
in  the  laboratory  (§  73),  the  shifting  of  the  lines  can  be  measured. 
The  velocity  of  approach  or  recession  of  the  star  can  then  be  com- 
puted, due  allowance  being  made  for  the  earth's  motion. 

The  majority  of  the  stars  yet  observed  in  this  way  exhibit  velocities 
of  less  than  thirty  miles  a  second.  The  best  modern  results  in  this 
line  of  work  are  not  subject  to  errors  exceeding  a  mile  a  second. 

361.  The  Sun's  Path.  —  A  man  is  in  a  boat  on  a  small  lake  sur- 
rounded by  a  forest.  The  boat  is  drifting,  he  knows  not  whither. 
He  watches  the  trees  carefully,  and  finally  perceives  that  the  trees 
at  his  right  appear  to  be  spreading  apart  and  growing  taller.  Those 
on  the  left  seem  to  be  crowding  more  closely  together.  He  can  de- 
tect no  change  in  the  relative  situations  of  the  trees  ahead  of  him,  or 
those  behind  him.  He  at  once  decides  that  his  boat  is  drifting 
toward  the  right. 

In    this    manner   astronomers    have  discovered    the   direction  in 
which  the  sun  with  its    attendant   planets    is    drifting.     While   the 


THE    FIXED    STARS.  24! 

proper  motions  of  the  stars  are  in  all  directions,  when  we  combine 
large  numbers  of  them  in  a  single  discussion,  a  prevailing  common 
drift  comes  out  clearly.  The  stars  in  the  constellations  Lyra  and 
Hercules  are  slowly  separating  from  one  another.  Those  in  the 
opposite  part  of  the  sky  are  crowding  together.  The  proper  mo- 
tions are  so  small  that  it  is  not  possible  to  fix  the  point  toward 
which  we  are  moving  with  much  precision.  Spectroscopic  observa- 
tions of  the  velocities  of  stars  make  the  sun's  velocity  only  from  8  to 
12  miles  a  second. 

362.  The  Central  Sun.  —  There  is  a  persistent  idea  that  there  is  a 
central  sun.     One  theory,  which  has  obtained  a  wide  currency,  is 
that  Alcyone,  the  brightest  of  the  Pleiades,  is  the  central  sun.     This 
theory  arose  fifty  years  ago  from  a  study  of  the  proper  motions  of 
the  Pleiades.     In   the  light   of  our  present  knowledge    concerning 
proper  motions,  the  theory  is  considered  untenable. 

The  hypothesis  that  the  sun  is  sweeping  around  a  gigantic  curve 
is  a  reasonable  one ;  but  no  deviation  from  a  straight  line  has  yet 
been  detected  in  its  motion.  Even  if  the  centre  of  its  motion  be 
found,  it  by  no  means  follows  that  all  the  other  bodies  in  the  uni- 
verse move  about  that  centre. 

363.  The  System  of  the  Stars.  —  Evidences  of  order  and  obedience 
to  law  are  so  numerous  in  the  entire  domain  of  physical  science, 
that  the  human  mind  instinctively  seeks  for  some  law  or  set  of  laws, 
in  accordance  with  which  all  the  stars  pursue  their  journeyings. 

The  only  law  now  known,  which  the  motions  of  the  heavenly 
bodies  follow,  is  that  of  gravitation.  But  while  there  are  many 
systems  more  or  less  similar  to  the  solar  system  among  the  stars, 
each  is  so  far  from  its  neighbors  that  it  experiences  very  little  attrac- 
tion from  them.  They  exist  in  great  variety,  from  the  largest  and 
most  complicated  clusters,  down  to  simple  double  stars ;  their  con- 
nection with  one  another  is  only  a  matter  of  conjecture.  It  seems 
very  probable  that  there  is  no  central  sun,  or  even  central  point, 
about  which  the  universe  moves  in  orderly  fashion. 

The  solar  system  is  a  fairly  well  regulated  family.  The  stellar 
system  seems  to  be  made  up  of  families  and  tribes  which  are  largely 
independent;  while  each  family  or  tribe  exercises  some  influence 
upon  the  neighboring  ones,  it  apparently  attends  pretty  strictly  to  its 
own  affairs. 

16 


242 


DESCRIPTIVE    ASTRONOMY. 


DOUBLE    AND    MULTIPLE    STARS. 

364.  Appearance  to  the  Naked  Eye.  —  By   surveying  the    heavens 
for  a  few  minutes  one  may  find  several  places  where  two  stars  lie  in 
close  proximity  to  each  other.     Theta  Tauri,  in  the  head  of  the  Bull, 
and  Alpha  Capricorni,  are  good  examples.     But  neither  of  these  is 
ordinarily  classed  as  a  double  star,  for  the  components  of  each  pair 
are  too  far  apart.     The  stars  which  make  up  a  real  "  double  "  are  so 
close  together  that  a  telescope  is  required  to  separate  them. 

365.  Appearance  through  a  Telescope.  —  Fig.  1 79  exhibits  some  of 
the  double  stars,  when  seen  under  a  high  magnifying  power.     When 


GAMMA    LEONIS 


ALPHA   HERCULIS 


BETA    CYGNI 


ANTARE5 


Fig.  179.  —  DOUBLE  STARS. 

the  two  stars  are  of  the  same  brightness  they  are  also  of  the  same 
color.  When  they  differ  considerably  in  brightness  the  smaller  star 
is  apt  to  have  a  bluish  cast.  Beta  Cygni,  in  the  foot  of  the  Cross,  is 
one  of  the  finest  of  those  colored  doubles  which  can  be  seen  with  a 
small  telescope.  The  large  star  is  reddish  yellow,  the  small  one 
greenish  blue.  The  colors  may  be  seen  beautifully,  by  putting  the 
stars  out  of  focus.  The  two  stars  are  often  so  close  to  each  other 
that  even  a  very  powerful  telescope  shows  them  bunched  together 


THE    FIXED    STARS. 


in  an  oblong  mass  of  light.     At  times  the  blaze  of  a  bright  star  quite 
overpowers  the  feeble  light  of  its  faint  companion. 

366.  Numbers  and  Nomenclature. — The  number  of  doubles  thus 
far  catalogued  is  over  10,000.     New  ones  are  being  discovered  con- 
tinually, but  not   at  a  rapid  rate,  since  there  are  few  stars   above 
the  eighth  magnitude  which  have  not  been  scrutinized  carefully  with 
large    instruments.     Doubles,  the  principal  stars   of  which  are   no 
brighter  than  the  ninth  magnitude,  are  rarely  catalogued. 

Each  double  retains  its  ordinary  name,  such  as  Sirius,  Gamma 
Virginis,  61  Cygni,  etc.,  and  acquires  an  additional  one  taken  from 
the  name  of  the  discoverer.  Thus  h  1064  is  one  of  Sir  John 
Herschel's  discoveries :  ft  462  was  found  by  Prof.  S.  W.  Burnham 
of  Chicago,  the  greatest  living  double-star  astronomer. 

367.  Optical  Double  Stars.  —  An    optical    double  star  is  one  the 
components  of  which  seem  to  be  near  each  other,  but  are  not;   one 
of  the  stars  lies  far  beyond  the  other.     Optical  pairs  form  but  a  very 
small  percentage  of  doubles.     They  are  detected  by  the  absence  of 
such  motion  as  would  ensue  were  the  stars  so  near  together   that 
their  mutual  attraction  caused  relative  motion. 

368.  Physical  Double  Stars.  —  The  stars  forming  a  physical  double 
are  subject  to  the  sway  of  their  own  mutual  attraction.     Observa- 
tions of  them    reveal   the  fact  that  they  move  in  elliptical  orbits. 
This  leads  to  the  belief  that  gravity  is  the  force  which  controls  their 
motions.     A  force  acting  according  to  some  other  law  might  pro- 
duce elliptical  motion,  as  is  proved  in  works  on  mechanics.     But 
since  the  spectroscope  shows  that  the  stars  are  composed  of  much 
the  same  materials  as  the  sun,  it  is  reasonable  to  suppose  that  their 
attractions  for  one  another  follow  the  same  law  which  holds  good 
in  the   solar  system.     Gravitation  may  therefore   be   considered  as 
universal. 

Physical  double  stars  are  usually  termed  binaries.  Many  of  the 
periods  of  revolution  are  hundreds  of  years  in  length ;  a  few  are  less 
than  a  year.  Some  of  the  orbits  are  several  times  as  large  as  that  of 
Neptune.  Others  are  smaller  than  that  of  Mercury. 

369.  Spectroscopic  Binaries.  —  When  the  components  of  a  binary 
are   so    close   together   that   the  most   powerful   telescopes   fail  to 
separate  them,  or  to  give  any  indication  that  the  star  is  double,  the 
spectroscope  in  a  few  instances  has  revealed  the  duplicity.     The  star 


244  DESCRIPTIVE    ASTRONOMY. 

Mizar  at  the  crook  of  the  handle  of  the  Great  Dipper  is  a  case  in 
point.  A  small  telescope  easily  resolves  this  into  two  stars.  Prof. 
E.  C.  Pickering,  in  1889,  found  by  his  photographs  of  this  double, 
that  the  spectrum  of  the  brighter  component  exhibited  strange 
anomalies.  At  regular  intervals  of  a  few  weeks  the  dark  lines  in  the 
spectrum  were  doubled.  The  explanation  of  this  depends  upon 
the  principle  (§  360)  that  when  a  star  is  approaching  us  the  lines  of 
its  spectrum  are  shifted  toward  the  violet,  and  when  it  is  receding 
the  lines  are  shifted  toward  the  red.  When  two  bright  stars,  close 
together  and  composed  of  the  same  substances  so  that  they  give 
the  same  spectra,  are  revolving  about  their  common  centre  of  gravity 
in  an  orbit  the  plane  of  which  is  nearly  edgewise  to  us,  one  star  will 
be  approaching  when  the  other  is  receding.  Were  the  stars  at  rest, 
their  spectra  would  coincide  in  position.  But  when,  on  account  of 
their  motion,  the  lines  in  one  spectrum  are  shifted  in  one  direction, 
and  those  in  the  other  in  the  opposite  direction,  the  lines  which 
formerly  coincided  will  appear  side  by  side. 


EARTH 


Fig.  180.  —  A  SPECTROSCOPIC  BINARY. 

When  the  stars  are  at  A  and  B  in  Fig.  180,  they  are  neither 
approaching  the  earth  nor  receding  from  it  so  far  as  their  orbital 
motion  is  concerned. 

The  two  close  stars  in  Mizar  complete  a  revolution  in  one  hun- 
dred and  four  days,1  in  an  orbit  of  the  same  size  as  that  of  Mars. 

Spica,  in  Virgo,  is  a  yet  more  wonderful  double.  The  compo- 
nents are  only  about  6,000000  miles  apart,  and  complete  a  revolution 
in  four  days. 

370.  Sirius.  —  Certain  minute  movements  of  Sirius  on  the  face  of 
the  sky,  hither  and  thither,  were  for  a  long  time  a  source  of  perplex- 
ity to  astronomers.  Fifty  years  ago  the  illustrious  German  astron- 
omer, Bessel,  announced  that  the  observations  of  Sirius  indicated 
that  it  was  describing  a  tiny  ellipse.  He  also  advanced  the  theory 
that  the  motion  was  caused  by  the  proximity  of  a  companion.  Less 

1  Possibly  in  just  half  that  time. 


THE    FIXED    STARS. 


245 


than  ten  years  thereafter,  two  other  German  astronomers  declared, 
as  the  result  of  an  elaborate  investigation,  that  the  period  of  orbital 
revolution  of  Sirius  and  his  unseen  satellite  was  fifty  years ;  they  also 
pointed  out  the  direction  in  which  the  companion  lay  from  the  larger 
star,  and  the  direction  of  its  motion.  Eight  years  later,  when  the 
Clarks  were  testing  an  i8|-inch  object-glass,  they  turned  it  upon 
Sirius.  The  keen  eye  of  Alvan  Clark,  Jr.  quickly  detected  a  faint 
star  in  the  blaze  of  light  surrounding  the  large  star.  It  was  soon 
found  to  be  moving  in  the  way  predicted.  The  mass  of  the  system 
is  six  times  that  of  the  sun.  The  faint  star,  which  gives  less  than 
10o00  as  much  light  as  the  main  star,  may  contain  one  third  of  the 
mass  of  the  system. 

371.  Planetary  Systems.  —  As  the  sun  is  the  ruler  of  a  planetary 
system,  many  of  the  stars  may  be  centres  about  which  troops  of 
planets  roll  and  shine.     Such  planets,  in  order  to  be  discovered  by 
us,  must  be  much  more  brilliant  in  comparison  with  their  suns  than 
are  those  of  the  solar  system.     Jupiter  himself,  if  searched  for  from 
Alpha  Centauri  with  the  most  powerful  telescope  ever  constructed 
by  man,  would  elude  the  most  searching  scrutiny.     Professor  Young 
has  computed  that  a  refracting  telescope  ten  feet  in  aperture  would 
be  needed.     Such  companions,  if  not  too  near  their  primaries,  may 
in  the    future  impress  themselves  on  photographic  plates  of  great 
sensitiveness. 

372.  Multiple    Stars.  —  Epsilon    Lyrae    is   a   fine    specimen    of   a 
multiple  star.     It  is  one  of  the  two  fourth  magnitude  stars  near  Vega 


EPSILON    LYRAE       THETAORIONI5         ZETA  CANCRI 
Fig.  181.  —  MULTIPLE  STARS. 

which  form  with  it  an  equilateral  triangle.  To  a  good  eye  the  star 
appears  oblong;  a  keen  eye  separates  it  into  two.  An  opera-glass 
shows  them  finely.  A  telescope  three  inches  or  more  in  aperture 


246  DESCRIPTIVE    ASTRONOMY. 

reveals  each  star  as  a  double.  We  have,  therefore,  a  quadruple 
star.  Each  pair  is  a  binary  :  it  is  probable  that  the  two  pairs  revolve 
about  their  common  centre  of  gravity,  completing  a  single  revolution 
in  many  thousands  of  years. 

Theta  Orionis  is  composed  of  six  stars.  It  is  located  in  the 
sword-handle  of  Orion,  and  is  involved  in  the  great  nebula  of  Orion 
(§  39°)  •  There  is  good  evidence  that  these  stars  have  been  formed 
by  the  condensation  of  a  portion  of  the  nebula. 

Zeta  Cancri  is  visually  a  triple  star,  two  being  close  together,  the 
other  farther  away.  The  close  pair  is  a  binary,  and  the  third  star 
apparently  revolves  about  the  binary,  but  with  singular  irregularities 
of  motion.  The  irregularities  are  thought  by  some  astronomers  to 
be  due  to  a  fourth  star  near  by,  but  invisible.  The  system  is  in 
that  case  composed  of  two  binaries,  which  revolve  about  their  com- 
mon centre  of  gravity. 

VARIABLE   STARS. 

373.  Definition :  Number :  Names.  —  Variable   stars  are   those   the 
brightness  of  which  has  been  observed  to  change.     Those  that  repeat 
the  same  series  of  changes  over  and  over  are  known  as  periodic,  the 
period  being  the  time  required  for  the  star  to  pass  through  one  com- 
plete cycle  of  change.     Some  naked  eye  stars  become  too  faint  to 
be  seen  with  a  telescope  four  inches  in  aperture.     New   variables 
are    discovered    from    time  to  time,    and   the  number    now  (1896) 
well  authenticated  is  400.     This    number  does   not    include   those 
variables  which  were  discovered   in    I895,1   in  certain  globular  star 
clusters;   nearly  100  variables  were  noted  in  a  single  cluster.    When 
such  stars  already  have  names  (§  341)  other  than  mere  numbers  in 
some   star  catalogue,  no  new  name  is  added  to  denote  variability. 
But  when  the  stars  are  faint,  so  that  they  have  not  received  such 
names,  the  first  such  variable  discovered  in  the  constellation  An- 
dromeda, for  instance,  is  named  R  Andromedae.     The  second  would 
be  S,  and  so  on  through  the  alphabet.     After  the  letter  Z  has  been 
reached,  further  discoveries  receive  the  designations  RR,  RS,  etc. 

374.  Classes.  —  These  are  classified  in  five  groups. 

Class  I.  embraces  temporary  stars,  which  suddenly  experience  an 
enormous  increase  in  brightness,  and  then  fade  away  gradually. 

1  By  Prof.  S.  I.  Bailey,  at  Arequipa,  Peru. 


THE    FIXED    STARS. 


247 


Class  II.  includes  periodic  stars  which  suffer  great  variations  of 
light  in  not  less  than  several  months. 

In  Class  III.  are  found  stars  which  exhibit  slight  irregular  fluctua- 
tions of  brightness. 

For  Class  IV.  are  taken  those  stars  of  short  periods,  the  light  of 
which  varies  smoothly  and  regularly. 

Class  V.  is  devoted  to  those  periodic  stars  which  suffer  a  remark- 
able diminution  of  light  for  a  few  hours,  every  few  days.  They  be- 
have as  if  temporarily  partially  eclipsed. 


Fig.  182.  —  TYCHO  BRAHE. 

375.  Temporary  Stars.  —  One  evening,  in  November,  1572,  when 
Tycho  Brahe  was  taking  his  usual  walk,  he  perceived  in  the  con- 
stellation of  Cassiopeia  a  new  star,  brighter  than  Sirius,  and  com- 
parable with  Venus  at  her  best.  Doubting  the  evidence  of  his 
eyes,  he  called  the  attention  of  several  others  to  the  splendid  object. 


248  DESCRIPTIVE    ASTRONOMY. 

For  some  days  the  star  could  be  discerned  in  full  daylight,  and  at 
night  shone  through  light  clouds  which  obscured  all  other  stars.     In 
December  it  began  to  wane  ;   at  the  end  of  that  month  it  had  become 
fainter  than  Jupiter.     Finally,  in  March,   1574,  it  disappeared  from 
view.     There  were  no  telescopes  then  to  watch 
it   further.     Its    color  changed  from  white  to 
yellow  and  red  successively,   and  returned  to 
_  white  before  it  faded  from  vision. 

STAR  Tycho  determined  its  place,  but  his  obser- 

•          •  vations   are   so   rude,  from   lack   of  telescopic 

aid,   that  it  is  impossible   to   tell  whether  the 
9  •  star  was  any  one  of  half  a  dozen  now  visible 


Fig.zSj.-TYCHO-SSTAR 

IN  CASSIOPEIA.  Nova  Aurigae  (§381)  belongs  to  this  class 

of  stars. 

376.  Mira.  —  Class  II.  is  fitly  represented  by  Mira,  which  is 
Omicron  Ceti.  The  period  of  this  star  is  eleven  months.  Most  of 
the  time  it  is  invisible  to  the  unassisted  eye,  but  once  .during  its 
period  it  rises  to  its  maximum  brightness,  which  Varies  from  the 
second  to  the  fifth  magnitude,  remains  there  about  a  week,  and  then 
sinks  more  slowly  back.  The  rise  and  fall  together  consume  about 
one  hundred  days.  During  the  remainder  of  its  period  it  is  of  about 
the  ninth  magnitude,  and  can  therefore  always  be  seen  with  a  good 
field-glass.  It  is  visible  to  the  naked  eye  about  six  weeks,  when 
near  its  maximum. 

377.  Class  III.  —  To  this  belong  Alpha  Orionis  and  Alpha  Cassio- 
peiae.  Alpha  Orionis  is  the  bright  reddish  star  in  the  shoulder  of 
Orion.  The  amount  of  fluctuation  is  small,  and  no  period  or  regu- 
larity of  fluctuation  has  been  found. 

378.  Beta  Lyrae.  —  This  star  is  a  good  example  of  Class  IV.     It  is 
of  the  fourth  magnitude,  and  varies  half  a  magnitude  on  each  side  of 
this.     The  period  is  nearly  thirteen  days  ;   during  this  time  the  star 
first  reaches  a  maximum  of  the  3.4  magnitude,  then  sinks  to  the  3.9 
magnitude,  next  rises  again  to  the  3.4  magnitude,  and  finally  sinks  to 
the  4.5  magnitude.     These  changes  are  thought  to  be  due  in  some 
way  to  the  action  of  one  or  more  companions,  revolving  about  the 
main  star. 

379.  Algol.  —  Beta  Persei  was  called  by  the  Arabians  Algol,  which 


THE    FIXED    STARS.  249 

means  the  Demon  Star;  they  had  therefore  undoubtedly  noticed 
the  variation  of  its  light.  Its  mean  period  has  been  very  accurately 
determined,  and  is  given  by  Chandler  as  2  d.  20  h.  48  m.  55  sec. 
During  most  of  the  time  it  is  of  the  second  magnitude.  Its  variation 
occupies  ten  hours,  the  magnitude  falling  to  the  fourth,  remaining 
there  for  twenty  minutes,  and  rising  again  to  the  normal  amount. 

ALGOL       GAMMA 
• —  -«.. 

ANDROMED"AL. ,    B£TA 

'"*•• -?'''    SQUARE  OF      \ 

ANDROMEDAE.  •-. 


V 
Fig.  184.  —  How  TO  FIND  ALGOL. 

The  cause  of  this  sudden  diminution  of  light  has  long  been  sus- 
pected to-  be  the  presence  of  a  dark  star  revolving  about  Algol  and 
partially  eclipsing  it  at  each  revolution.  The  truth  of  this  has  re- 
cently been  rendered  nearly  certain  by  spectroscopic  observations 
(§  360)  which  show  that  Algol  alternately  approaches  us  and  recedes, 
just  as  if  it  were  one  component  of  a  binary  system.  The  following 
approximate  data  concerning  this  binary  have  been  derived :  — 

Diameter  of  the  principal  star,  i,ooopoo  miles. 

Diameter  of  the  dark  companion,  800,000      " 

Distance  between  their  centres,  3,000,000      " 

Velocity  of  the  companion,  55  miles  per  second. 

Mass  of  the  principal  star,  f  of  the  sun's  mass. 

Mass  of  the  companion,  f  of  the  sun's  mass. 

There  are  certain  small  inequalities  in. the  period  of  variability 
which  Chandler  explains  by  the  theory  that  the  binary  already 
mentioned  is  involved  in  an  orbital  revolution  with  a  heavy  faint 
star,  in  a  period  of  about  130  years.  The  size  of  this  new  orbit  is 
about  equal  to  that  of  the  orbit  of  Uranus.  It  is  possible  that  there 
are  other  bodies  in  the  system.  Algol  belongs  to  Class  V. 

380.  Y  Cygni. — This  star  is  one  of  the  most  interesting  of  vari- 
ables :  it  belongs  to  the  Algol  type.  Twice  in  every  three  days  it  has 


250  DESCRIPTIVE    ASTRONOMY. 

a  minimum,  at  which  the  light  is  one  half  of  the  maximum  amount. 
This  fluctuation  is  explained  by  the  supposition  that  the  star  is  'a 
close  double,  the  two  components  being  equal  in  size  and  brightness, 
and  revolving  in  a  plane  which  is  turned  edgewise  to  us.  Twice 
in  each  revolution  about  their  common  centre  of  gravity  one  star 
eclipses  the  other :  the  period  of  revolution  is  thus  72  hours.  If 
we  consider  several  successive  minima,  calling  them  first,  second, 
third,  etc.,  we  find  that  the  time  from  the  first  to  the  third  is  72 
hours,  as  is  also  the  interval  between  the  second  and  fourth,  but  the 
interval  from  the  first  to  the  second  is  not  36  hours  as  would  be  ex- 
pected. The  interval  between  the  first  and  second  minima  may  be 
32  hours,  for  instance :  then  40  hours  would  elapse  between  the 
second  and  third,  and  the  interval  between  the  third  and  fourth 


LINE   OF   VISION 


Fig.  185.  — Y  CYGNI. 

would  be  32  hours  again.  This  irregularity  is  accounted  for  by 
the  assumption  that  the  stars  revolve  in  ellipses  which  lie  "  broad- 
side "  to  our  line  of  vision,  as  shown  in  Fig.  185.  When  the  stars 
are  at  A  and  B  respectively,  there  is  an  eclipse  or  minimum.  Be- 
tween the  first  and  second  minima  the  stars  are  describing  the  short 
parts  (ACB  and  BDA)  of  their  orbits.  Between  the  second  and 
third  minima  the  long  portions,  BEA  and  AFB,  are  described. 

This  simple  explanation  does  not  wholly  account  for  the  observed 
irregularities  in  the  times  of  the  eclipses.  In  1886  each  period  was 
36  hours,  while  in  1891  the  successive  intervals  were  respectively  31 


THE    FIXED    STARS.  25! 

hours  and  43  hours.  This  anomaly  can  be  explained  upon  the  hy- 
pothesis that  the  ellipses  are  shifting  their  position  with  reference  to 
our  line  of  sight,  the  disturbance  being  due  to  the  attraction  of  some 
neighboring  unseen  body. 

381.  Nova  Aurigse. —  Nova  Aurigae  was  discovered  in  the  latter 
part  of  January,  1892,  by  Dr.  Thomas  D.  Anderson  of  Edinburgh,  an 
amateur  astronomer,  who  was  in  the  habit  of  observing  with  a  hand 
telescope  magnifying  only  ten  diameters.  It  was  of  the  fifth  mag- 
nitude. Soon  astronomers  all  over  the  world  were  observing  the 
spectrum  and  changes  of  brightness  of  this  new  star.  The  question 
at  once  arose  whether  it  had  previously  been  a  telescopic  star,  and 
when  it  first  displayed  itself.  Fortunately  photographs  of  the  re- 
gion of  sky  in  which  it  lay  were  at  hand.  On  Dec.  10,  1891,  six 
weeks  before  its  visual  discovery,  it  had  impressed  itself  on  one  of 
the  photographic  plates  exposed  at  the  Harvard  College  Observa- 
tory. A  photograph  taken  in  Europe  on  December  8  showed  no 
trace  of  the  star.  The  photographic  evidence  shows  that  it  was 
somewhat  brighter  in  the  latter  part  of  December,  1891,  than  a  month 
later,  when  the  attention  of  astronomers  was  called  to  it. 

The  spectrum  was  found  to  be  of  bewildering  complexity ;  there 
were  fine  bright  lines  and  broad  dark  ones ;  some  of  the  lines  were 
shifted  in  one  direction,  and  others  in  the  opposite,  as  if  there  were 
two  bodies  moving  in  widely  different  directions.  In  a  few  weeks  it 
began  to  decline  in  brightness  rapidly.  On  April  24  it  was  only  of 
the  sixteenth  magnitude,  and  two  days  later  it  was  hardly  percepti- 
ble with  the  Lick  telescope. 

The  complexity  of  its  spectrum  led  to  the  greatest  variety  of 
theories  as  to  the  cause  of  the  outburst.  By  some  it  was  attributed 
to  the  near  approach  of  two  bodies  moving  with  immense  speed  ; 
their  proximity  caused  great  mutual  disturbances  of  a  tidal  nature, 
leading  to  the  production  of  enormous  eruptions  similar  to  solar 
prominences,  though  on  a  vastly  greater  scale. 

Another  hypothesis  was,  that  some  unknown  heavenly  body, 
speeding  along  its  far  distant  path,  came  into  collision  with  a  cloud 
of  cosmical  matter,  similar  to  the  meteoric  aggregations  encountered 
by  the  earth,  but  much  denser.  Photography  has  revealed  the  pres- 
ence of  such  clouds  (§  338)  in  the  Milky  Way,  and  the  Nova,  like 
most  temporary  stars,  was  situated  in  the  Galaxy. 


252  DESCRIPTIVE    ASTRONOMY. 

But  another  strange  chapter  is  to  be  added  to  the  history  of  this 
remarkable  object.  In  August  it  was  found  to  have  brightened  up, 
having  attained  the  tenth  magnitude.  It  then  appeared  like  a  small 
star  surrounded  by  a  nebulous  atmosphere,  and  its  spectrum  was  that 
of  a  planetary  nebula  (§  385).  It  still  (1896)  retains, this  appearance. 
It  is  not  improbable  that  this  wonderful  object  is  at  so  stupendous 
a  distance  that  all  these  changes  occurred  before  the  astronomers 
who  have  observed  them  were  born. 

The  amount  of  light  given  out,  when  the  Nova  was  at  its  best, 
may  have  been  many  times  greater  than  that  radiated  by  the  sun. 

382.  Causes  of  Variability.  —  There  have  been  many  theories  upon 
this   topic.     The  variability  of  stars  of  the  Algol  type  is  well  ex-- 
plained by  the  hypothesis  of  eclipses  by  unseen  bodies  revolving 
about  the  variables. 

The  behavior  of  many  variables  can  be  explained  by  the  hypoth- 
esis that  they  have  spots,  like  the  sun's,  though  much  larger, 
and  that  these  spots  have  their  times  of  maximum  and  minimum 
frequency,  as  do  the  solar  spots.  If  a  star  had  one  or  more  large 
companions  revolving  about  it,  their  attractions  might  cause  consid- 
erable tidal  disturbances,  which  would  give  rise  to  variability. 

The  sudden  appearance  of  temporary  stars  may  be  explained  by 
terrific  outbursts  of  heated  vapors,  analogous  to  the  solar  prom- 
inences. Lockyer  has  advanced  the  theory,  that  the  variables  are 
not  single  masses,  but  are  rather  compact  swarms  of  meteoric 
bodies,  attended  by  satellite  swarms  revolving  in  very  eccentric  or- 
bits. The  satellite  swarms  are  supposed,  when  nearest  the  main 
swarm  once  in  every  revolution,  to  collide  with  its  outlying  meteors, 
thus  producing  a  temporary  increase  of  light. 

Much  research  must  yet  be  made  before  the  complex  phenomena 
exhibited  by  variable  stars  can  receive  any  adequate  explanation. 

383.  How  to  Observe  Variables.  —  The  observations  of  the  varia- 
tions, of  these  stars  in  brightness  often  do  not  require  the  use  of 
any  telescope  larger  than  an  opera-glass.     When  a  star  is  near  its 
maximum  or  minimum,  it  is  compared  with  adjoining  stars  of  nearly 
the  same  brightness :   it  is  noted  as  being  equal   in  brightness  to 
some  particular  one  of  its  neighbors,  or  a  trifle  brighter  or  fainter 
than  others,  at  a  given  time.     The  object  of  the  observations  is  to 
determine  the  time  of  maximum  or  minimum  brightness. 


THE    FIXED    STARS.  253 

The  approximate  times  are  given  in  various  publications,1  for  the 
observer's  guidance.  Most  of  the  observations  made  in  this  country 
on  these  interesting  objects  are  by  amateur  astronomers. 


EXERCISES. 

384.  i.  With  an  opera-glass  or  spy-glass  look  at  some  portion 
of  the  sky,  which  appears  to  the  naked  eye  to  be  barren  of  stars,  and 
count  the  number  in  the  field  of  view. 

2.  On  a  night  when  the  moon  is  not  shining,  direct  an  opera-glass 
or  spy-glass  toward  some  bright  spot  in  the  Milky  Way,  and  find 
out  whether  the  light  from  that  particular  locality  is  due  to  a  number 
of  faint  stars,  or  to  a  few  brighter  ones. 

3.  On  a  night  when  the  moon  is  not  shining,  find  a  dark  spot  in 
the  Milky  Way,  and   make   a  drawing  showing  its   location  among 
the  stars. 

4.  Observe  five  of  the  brightest  stars  visible  at  any  given  hour, 
and  write  down  the  name  of  each,  together  with  its  color. 

5.  Estimate  the  color  of  Zeta  Ursae  Majoris  (Mizar),  and  of  the 
minute  star  (Alcor)  close  by  it. 

6.  What  is  the  origin  of  the  name  Groombridge  1830?    (§  341.) 

7.  Count  the  stars  visible  to  the  naked  eye  inside  the  bowl  of 
the  Great  Dipper,  when  the  moon  is  not  shining,  and  the  Dipper  is 
not  low  down. 

8.  The  light  of  a  sixth  magnitude  star  is  equivalent  to  the  com- 
bined light  of  how  many  of  the  eighth  magnitude? 

9.  If  a  cluster  were  spherical  in  form,  and  the  stars  distributed 
uniformly  through  it,  would  it  appear  to  be  more  condensed  near 
the  centre  than  at  the  edge  ? 

10.  The  intensity  of  light  varies  inversely  as  the  square  of  the 
distance ;   that  is,  if  two  equal  lights  are  at  distances  of  one  mile 
and  three  miles  from  the  eye,  the  farther  one  would  not  look  one 
third  as  bright,  but  one  ninth  as  bright.     If  a  given  star  were  placed 
at  half  its  present  distance  from  us,  it  would  look  how  many  times 
as  bright  as  before? 

1  The  times  of  minima  of  variables  of  the  Algol  type  are  given  in  "  Popular  Astron- 
omy," every  month.  For  methods  of  observation,  see  articles  by  Mr.  J.  A.  Parkhurst  in 
"  Popular  Astronomy  "  for  December,  1893,  an^  January,  1894. 


254  DESCRIPTIVE    ASTRONOMY. 

1 1.  The  semidiameter  of  the  earth's  orbit  being  93,000000  miles, 
how  far  off  is  a  star  which  has  an  annual  parallax  of  one  tenth  of  a 
second  of  arc  ? 

12.  Show  that  the  time  required  for  light  to  come  to  us  from  a 
star  having  a  parallax  of  one  hundredth  of  a  second  of  arc  is  over 
three  hundred  years. 

13.  Do    Sirian    stars    have    atmospheres    of    large    absorptive 
power? 

14.  Do  the  spectra  of  solar  stars  indicate  that  they  are  probably 
more  dense  than  Sirian  stars  or  less  dense? 

15.  Do  the  spectra  of  the  Wolf-Rayet  stars  show  that  they  are 
surrounded  by  extensive  atmospheres,  which  absorb  the  rays  com- 
ing from  within?     (§  73.) 

1 6.  Show  that  the  sun,   if  removed    to   the   distance  of  Sirius, 
would  appear  to  be  less  than  one  fortieth  as  bright  as  the  latter. 

17.  If  the  stellar  system  were  in  the  form  of  a  sphere,  throughout 
which  the  stars  were  distributed  uniformly,  and  we  were  at  its  centre, 
would  the  stars  appear  to   be  uniformly  distributed  over  the  face  of 
the  sky? 

1 8.  Does  the  fact  that  there   are  many  more  stars  visible  when 
we  look  toward  the   Galaxy  than  when  we  look  in  other  directions 
indicate    that   the  stellar  universe  is  shaped    somewhat  like  a  thin 
cheese?      (In  answering    this  question,  assume   that   the  stars  are 
distributed   with  some   uniformity   through    the   space  which  they 
occupy.) 

19.  Might  the  appearance  of  the  Galaxy  be  accounted  for  on  the 
supposition  that  it  is  an  irregular  ring  of  closely  packed  stars  sur- 
rounding us? 

20.  Spectroscopic  observations  show 
that  a  star  is  approaching  us  at  the 
rate  of  thirty  miles  a  second,  and  visual 
observations  show  that  it  is  apparently 
moving  perpendicular  to  the  line  of 
sight  with  a  velocity  of  forty  miles  a 
second;  according  to  the  principles  of 


Fis- l86-  mechanics    its    real    velocity   is    repre- 

sented  by  the  diagonal  of  the  rectangle  in  Fig.  186.     Prove   that 
the  real  velocity  is  fifty  miles  a  second. 


THE    FIXED    STARS.  255 

21.  If  the  earth  and  a  certain  star  are  moving,  at  a  given  time, 
with  the  same  velocity  in  the  same  direction,  will  the  lines  of  the 
star's  spectrum  be  shifted  from  their  normal  place? 

22.  Give  some  reasons  why  the  stars  differ  in  brightness. 

23.  Examine  Theta  Tauri  with  the  naked  eye;   if  you  cannot  see 
it  double,  your  vision  is  defective. 

24.  If  the  orbit  of  a  certain  binary  were  a  perfect  circle,  and  a 
line  drawn  from  the  observer's  eye  to  either  of  the  stars  were  oblique 
to  the  plane  of  the  circle,  would  the  orbit  appear  to  us  to  be  a  circle 
or  an  ellipse? 

25.  If  the  plane  of  the  orbit  of  a  binary  passed  through  an  ob- 
server's eye,  would  one  of  the  stars  ever  occult  the  other,  if  they 
were  equal? 

26.  If  one  star  of  a  binary  is  more  massive  than  the  other,  to 
which  one  will  their  common  centre  of  gravity  lie  the  nearer? 

27.  If  one  component  of  a  binary  is   much   brighter  than  the 
other,  does  it  follow  that  it  is  more  massive? 

28.  If  the  earth  were  fixed,  and  the  plane  of  the  orbit  of  a  binary 
were  perpendicular  to  a  line  drawn  to  the  star  from  the  observer's 
eye,  would  the  spectroscope  enable  us  to  determine  the  velocity  of 
either  star? 

29.  As  binaries  revolve,  do  the  components  sometimes  appear 
closer  together  than  at  others  ? 

30.  If  the  plane  of  the  orbit  of  a  binary  passed  through  the  ob- 
server's eye,  how  would  the  star  appear  in  a  telescope,  when  one 
body  was  between  us  and  the  other? 

31.  What  does  the  name  Y  Cygni  signify? 

32.  What  is  the  signification  of  the  designation  Nova? 

33.  If  the  outburst  of  a  temporary  star  be  due  to  the  collision  of 
some  star  with  a  meteor-like  cloud  of  comparatively  small  bodies, 
why  does  it  gradually  fade  away? 

34.  Upon  the  collision  theory  how  can  the  reappearance  of  Nova 
Aurigae  in  August,  1892,  be  explained? 

35.  Suppose  Mira  to  be  a  dense  cluster  of  meteoric  bodies,  about 
which  another  cluster  is  revolving,  in  a  very  elliptical  orbit.     Could 
the  variability  of  Mira  be  accounted  for  by  the  hypothesis  of  a  peri- 
odic collision  between  Mira  and  its  companion? 

36.  Could  the  fact  that  Mira,  when  brightest,  may  be  anywhere 


256  DESCRIPTIVE    ASTRONOMY. 

from  the  second  to  the  fifth  magnitude,  be  explained  by  the  collision 
theory  advanced  in  the  preceding  exercise? 

37.  If  Y  Cygni  is  a  binary  consisting  of  two  stars  of  equal  size 
and  brightness,  will  its  minima  occur  when  one  of  the  stars  is  behind 
the  other? 

38.  If  the  ellipses  in  Fig.  185  lay  "endwise"  to  the  earth,  so  that 
our  line  of  vision  went  through  the  points  E  and  F,  would  each  in- 
terval between  successive  minima  of  Y  Cygni  be  36  hours? 

39.  If  the  ellipses  in  Fig.  185  did  not  lie  either  exactly  "  broad- 
side "  or  "  endwise  "  to   our  line  of  vision,  would  the  time  intervals 
between  successive  minima  of  Y  Cygni  be  equal? 

40.  Is   there   any  reason  not   mentioned  in    §  380  why   one  of 
the   stars  would  traverse  the  arc   BDA  of  Fig.    185   more  quickly 
than  the  arc  AFB? 


THE    NEBULA.  257 


CHAPTER   XIII. 

THE    NEBULA. 

"  This  world  was  once  a  fluid  haze  of  light, 
Till  towards  the  centre  set  the  starry  tides, 
And  eddied  into  suns,  that  wheeling  cast 
The  planets." 

TENNYSON. 

385.  Various  Forms.  —  Nebulae  are  cloud-like  objects  of  a  bewilder- 
ing variety  of  forms.     They  are  to  be  carefully  distinguished  from 
clusters,  which  are  aggregations  of  stars.     A  true  nebula  does  not 
consist  of  separate  stars.     Many  clusters,  however,  have  nebulous 
matter  associated  with  them,  and  many  nebulae  contain  stars  within 
their  borders.     A  large  nebula  is  in  general  of  an  irregular  shape. 
In   it  are   to  be  seen   many  spots   brighter,  and   presumably  more 
condensed,  than  the  rest  of  the  nebula :   there  are  also  found  dark 
spots,  rifts,  and  streams  of  various  shapes.     The  cuts  of  the  nebulae 
of  Orion  and  Andromeda  (Figs.   189  and   190)  illustrate  these  pe- 
culiarities. 

Spiral  nebulae,  of  which  there  are  several,  exhibit  convolutions 
like  those  of  the  hair-spring  of  a  watch ;  the  appearance  resembles 
that  of  a  Catherine  wheel. 

Annular  nebulae  are  ring-shaped  objects,  darker  in  the  centre 
than  at  the  edge. 

Planetary  nebulae  have  small  round  disks  of  approximately  uniform 
brightness  throughout.  They  are  usually  brightest  in  the  centre. 

A  nebulous  star  has  a  strong  central  condensation,  surrounded 
by  a  nebulous  envelope.  It  is  frequently  difficult  to  decide  whether 
an  object  should  be  called  a  planetary  nebula  or  a  nebulous  star. 

Double  and  variable  nebulae  are  known :  no  orbital  revolution 
has  been  detected  in  the  double  nebulae :  no  law  of  variation  is 
known  for  the  variable  ones. 

386.  Number,    Distance,    and   Grouping. — The    number   of  known 
nebulae  is  about  eight  thousand.     New  ones  are  continually  being 

17 


258  DESCRIPTIVE    ASTRONOMY. 

discovered,  especially  by  photography,  but  most  of  the  discoveries 
are  exceedingly  faint  and  uninteresting. 

No  nebula  has  yet  revealed  any  parallax  (§  350).  Yet  the  many 
and  intimate  associations  of  nebulae  with  stars  lead  to  the  belief  that 
they  are  at  the  same  distances. 


Fig.  187.  —  THE  PLEIADES:   PHOTOGRAPHED  BY  ROBERTS.! 

In  the  Pleiades,  photography  has  revealed  the  presence  of  a  mass 
of  nebulous  matter  surrounding  four  of  the  bright  stars,  and  con- 
nected with  another  by  a  faint  ray.  The  brightest  star,  and  some 
smaller  ones  near  it,  are  involved  in  a  similar  nebula.  Other  faint 
stars  in  the  vicinity  are  connected  by  wisps  of  nebulous  matter 
emanating  from  the  vicinity  of  one  of  the  bright  stars. 

1  An  English  amateur  astronomer. 


THE    NEBULAE.  259 

The  multiple  star  Theta  Orionis  (§  372)  lies  in  a  dark  space  in 
the  Great  Nebula  of  Orion,  the  four  brighter  stars  looking  like  eggs 
in  a  bird's  nest.  The  appearance  suggests  that  the  stars  are  con- 
densations formed  from  the  surrounding  nebulous  matter.  Further- 
more, certain  lines  in  the  spectrum  of  Theta  Orionis  are  matched  by 
corresponding  ones  in  the  spectrum  of  the  nebula. 

While  the  stars  are  crowded  together  in  the  vicinity  of  the  Milky 
Way,  the  majority  of  the  nebulae  lie  outside  of  it.  Their  law  of  distri- 
bution over  the  sky  is  opposite  to  that  of  the  stars  (§  346).  They  are 
most  numerous  where  the  stars  are  least  numerous,  and  vice  versa. 

387.  Sizes :  Changes  of  Appearance.  —  Nebulae  vary  greatly  in  size. 
Some  are  so  small  as  to  look  like  stars  in  a  small  telescope.     Others 
are  the  most  gigantic  objects  ever  revealed  to  the  eye  of  man.     The 
Great  Nebula  in  Orion,  as  recently   photographed,   covers  a   large 
part  of  the  entire  constellation. 

There  are  serious  discrepancies  between  old  drawings  of  some  of 
the  nebulae  and  recent  delineations  of  them.  Drawings  of  so  faint 
objects,  made  with  telescopes  of  different  sizes  and  under  widely 
different  circumstances,  would  naturally  fail  to  agree.  While  ma/iy 
of  the  apparent  changes  are  due  to  such  causes,  there  remains  a 
small  residuum  of  cases  which  cannot  be  explained  reasonably,  ex- 
cept on  the  hypothesis  that  real  changes  in  the  nebulae  have  taken 
place.  The  "  Trifid  "  Nebula  shown  in  Fig.  188  is  an  illustration  in 
point.  A  star,  which  was  located  in  one  of  the  dark  lanes  at  the 
opening  of  the  nineteenth  century,  is  now  involved  in  the  nebulous 
matter.  The  star  has  not  changed  its  position  with  respect  to  the 
neighboring  stars:  therefore  the  nebula  must  have  changed.  Such 
is  the  result  of  an  investigation  made  by  Professor  Holden,  Director 
of  the  Lick  Observatory. 

388.  Spestra. —  About  half  of  the  nebulae  give  spectra  containing 
bright  lines;   thus  showing  (§  73)  that  they  may  be  composed  of 
glowing  gas  under  low  pressure.     Four  of  these  lines  are  generally 
seen  without  difficulty  with    the  powerful  spectroscopes  now  em- 
ployed.    Two  of  them  demonstrate  the  presence  of  glowing  hydro- 
gen.     The    origin    of  the  other   two  is   unknown.      Besides    these 
characteristic  nebular  lines,  several  others  have  been  seen ;   by  some 
of  these,  helium  and  sodium  are  fairly  recognized.    The  Great  Nebula 
in  Orion  is  the  finest  specimen  of  this  class. 


26O  DESCRIPTIVE    ASTRONOMY. 

Most  of  those  nebulae  which  do  not  exhibit  bright-line  spectra 
give  continuous  spectra  (§  73)  simply.  They  may  be  composed  of 
gaseous  matter  under  high  pressure,  or  of  glowing  liquid  matter,  or 


Fig.  188.  — THE  TRIFID  NEBULA. 

of  a  mixture  of  both.  The  Nebula  in  Andromeda  belongs  to  this 
class:  it  is  plentifully  besprinkled  with  stars.  Incandescent  solid 
matter,  unenveloped  by  a  gas,  would  give  a  continuous  spectrum. 


THE    NEBULA. 


26l 


We  have  no  proof,  however,  that  matter  exists  in  that  form  any- 
where in  the  universe.  A  few  nebulse  give  both  continuous  and 
bright-line  spectra. 


Fig.  189.  —  THE  NEBULA  IN  ANDROMEDA  :   PHOTOGRAPHED  BY  ROBERTS. 


389.  The  Nebula  in  Andromeda.  —  This  nebula  is  plainly  visible  to 
the  naked  eye,  and  has  often  been  mistaken  for  a  comet.  It  has  a 
tolerably  regular  elliptical  outline,  and  a  strong  central  condensation. 


262  DESCRIPTIVE    ASTRONOMY. 

Fig.  189  gives  the  impression  that  it  is  surrounded  by  rings  like 
those  of  Saturn,  or  that  it  is  a  gigantic  spiral.  The  appearance  of 
this  nebula  is  very  interesting  in  its  relation  to  the  nebular  hypothe- 
sis (§  394),  that  all  stars  and  planets  were  formed  by  the  conden- 
sation of  nebulous  matter.  According  to  this  theory,  the  two 
condensations  outside  of  the  main  elliptical  portion  may  be  planets 


Fig.  190.  —  THE  NEBULA  IN  ORION:   DRAWN  BY  BOND  AT  THE 
HARVARD  COLLEGE  OBSERVATORY. 

in  the  process  of  formation.  In  August,  1885,  a  new  star  appeared 
close  to  the  nucleus  of  the  nebula ;  at  first  it  was  bright  enough  to 
be  seen  with  an  opera-glass,  but  it  faded  away  to  invisibility  in  a  few 
months.  Its  spectrum  was  almost  the  same  as  that  of  the  nebula ; 
hence  the  star  was  probably  in  the  nebula.  It  exhibited  no  sensible 
parallax. 


THE    NEBULA.  263 

390.  The  Great  Nebula  of  Orion.  —  In  the  sword-handle  of  Orion 
are  three  stars  in  a  line,  easily  seen  with  the  naked  eye.    The  central 
one  of  these  appears  hazy :   it  is  the  multiple  star  Theta   Orionis, 
shown  in  Fig.  190,  near  the  centre  of  the  nebula.     This  star  is  com- 
monly known  as  the  Trapezium,  because  the  four  brighter  stars  in  it 
form  that  geometrical  figure.     The  spectrum  of  one  of  these  stars 
has   been    photographed,   and  exhibits   bright  lines    corresponding 
to  lines   in  the  spectrum  of  the  nebula.     This   indicates   that   the 
star  is  a  sphere  of  nebulous  matter,  not  yet  condensed  as  much  as 
stars  ordinarily  are.     The  brightest  portion  of  the  nebula  is  in  the 
immediate  vicinity  of  the  multiple  star.     Thence  it  branches  off  in 
wonderful  forms,  which  contrast  beautifully,  in  their  delicate  tracery, 
with  the  blackness  of  the  adjacent  regions.     Photography  reveals  a 
vast  extension  of  the  nebulosity  which  the  most  powerful  telescopes 
fail  to  show. 

Keeler  has  determined  spectroscopically  that  the  nebula  is  retreat- 
ing from  us  at  the  rate  of  nearly  eleven  miles  per  second. 

If  the  moon  be  absent,  the  nebula,  even  in  a  small  telescope,  must 
call  forth  the  admiration  of  the  beholder.  It  is  the  finest  object  of 
its  class  in  the  heavens. 

391.  Other  Notable  Nebulae.  —The  Dumb-bell  Nebula  in  Vulpecula 
(between  Lyra  and  Delphinus)  appears  in  a  small  telescope  to  be 
composed  of  two  oval  masses  in  contact. 

The  Ring  Nebula  in  Lyra  is  situated  a  third  of  the  way  from  Beta 
to  Gamma  Lyrse.  It  is  the  only  one  of  its  kind  which  can  be  seen 
with  a  small  telescope,  and  is  shown  in  Fig.  191,  as  seen  in  a  1 5-inch 
telescope. 

Of  Spiral  Nebulae,  one  of  the  most  remarkable  is  the  one  in  Canes 
Venatici,  shown  in  Fig.  192.  It  is  three  degrees  from  the  star  in  the 
end  of  the  tail  of  the  Great  Bear.  The  appearance  is  as  if  a  slow 
rotation  were  taking  place. 

The  Trifid  Nebula  is  situated  in  Sagittarius :  it  is  distinguished 
by  the  curious  triple-pronged  dark  area,  which  gives  it  the  appear- 
ance of  being  cracked  open.  This  is  the  nebula  previously  men- 
tioned, which  affords  distinct  evidence  of  change.  It  is  shown  in 
Fig.  188. 

392.  Real  Form  of  Spiral  Nebulae.  — While  these  nebulae  exhibit  to 
the  eye,  more  or  less  perfectly,  the  appearance  described  in   §  385, 


264  DESCRIPTIVE    ASTRONOMY. 

this,  may  not  be  their  real  form,  since  we  see  simply  their  projec- 
tions on  the  sky.  In  1888  Professor  Holden  discovered,  at  the  Lick 
Observatory,  that  one  of  the  planetary  nebulae  had  a  spiral  filament 


Fig.  191.  —  THE  RING  NEBULA  IN  LYRA:  DRAWN  BY  BOND  AT  THE 
HARVARD  COLLEGE  OBSERVATORY. 

within  it.  This  led  him  to  a  study  of  the  best  extant  drawings  of 
the  spiral  nebulae.  He  found  that  their  various  forms  can  be  ex- 
plained on  the  assumption  that  the  filaments  which  give  the  spiral 
appearance  are  really  of  the  form  of  a  corkscrew.  He  bent  a  wire 


THE    NEBULA. 


265 


into  this  shape,  and  was  able,  by  holding  it  in  different  positions,  to 
represent  the  shapes  of  the  various  spirals  shown  in  the  drawings. 

393.  The  Magellanic  Clouds.  —  The  Magellanic  Clouds,  or  Nube- 
culae,  are  situated  in  the  southern  celestial  hemisphere,  and  are  not 
visible  in  middle  north  latitudes.  They  are  two  cloudy  masses  of 


Fig.  192.  —  THE  SPIRAL  NEBULA  IN  CANES  VENATICI  : 
PHOTOGRAPHED  BY  ROBERTS. 

light,  the  larger  one  of  which  has  an  area  about  equal  to  that  of  the 
bowl  of  the  Great  Dipper;  the  smaller  one  is  only  one  fourth  as 
large.  Both  are  plainly  visible  to  the  naked  eye,  and  resemble  por- 
tions of  the  Milky  Way.  They  exhibit  a  marvellous  structure  in  a 
telescope :  nebulae,  both  regular  and  irregular  in  form,  and  star  clus- 
ters of  all  degrees  of  condensation,  are  mingled  promiscuously.  The 
larger  cloud  contains  about  three  hundred  of  these  objects. 


266 


DESCRIPTIVE    ASTRONOMY. 


THE  NEBULAR   HYPOTHESIS. 

394.  General  Statement.  —  The  celebrated  Nebular  Hypothesis  is 
an  attempt  to  account  for  the  present  form  of  the  solar  system  by 
a  process  of  orderly  evolution.  Its  name  indicates  that  it  pre- 
supposes the  existence  of  a  nebulous  mass,  the  parent  of  the  well 
ordered  worlds  which  we  now  behold.  The  chaotic  mass  of  world 
stuff  may  be  described,  in  Milton's  words,  as 

"  A  dark, 

Illimitable  ocean,  without  bound, 

Without  dimension,  where  length,  breadth,  and  height, 
And  time  and  place,  are  lost ;  where  eldest  Night 
And  Chaos,  ancestors  of  Nature,  hold 
Eternal  anarchy,  amidst  the  noise 
Of  endless  wars,  and  by  confusion  stand." 

Various  writers  suggested,  and  partially  worked  out,  the  nebular 
hypothesis,  but  the  first  to  give  it  an  extensive  mathematical  devel- 
opment was  Laplace.  We  proceed  to  state  his  theory,  and  its  modi- 
fications, setting  forth  afterwards  the  facts  which  give  color  to  it. 

395.  Laplace's  Theory.  —  Accord- 
ing to  this  theory  the  original  neb- 
ula was  a  mass  of  intensely  heated 
gas,  which  had  by  reason  of  the 
mutual  attractions  of  its  particles 
assumed  a  globular  form,  and  had 
acquired  a  motion  of  rotation. 
As  its  heat  was  radiated  away,  the 
nebula  contracted,  and  rotated  more 
swiftly ;  the  mass  became  flattened 
at  the  poles,  and  when  the  "  cen- 
trifugal force  "  at  the  equator  bal- 
anced the  force  of  gravity  there, 
a  ring  of  equatorial  matter  was 
abandoned.  The  spheroid  left 
within  the  ring  rotated  still  more 
rapidly  until  another  ring  was  left 
behind,  etc.  The  matter  in  each  ring  gradually  condensed  into  a 
planet,  which  in  turn  rotated  and  abandoned  rings,  which  usually 


Fig.  193.  —  LAPLACE. 


THE    NEBULAE.  267 

condensed  into  satellites :  in  the  case  of  Saturn  some  of  the  rings 
failed  to  break  up  into  satellites  of  goodly  size. 

396.  Changes  in  the  Theory.  —  As  facts  and  laws  unknown  in  La- 
place's  day   have    been    discovered,    various    modifications    of  the 
original    theory   have    been   proposed.      It  is    no  longer   necessary 
to  suppose  that  the  parent  nebula  was  originally  at  a  high  temper- 
ature :    it  is  regarded   as  more  probable  that   it  was  a  cold  cloud 
of  finely  divided    matter,  which   became    heated  in  the  process  of 
contraction. 

Since  some  parts  of  the  parent  mass  were  probably  denser  than 
others,  it  is  not  likely  that  rings  were  usually  abandoned,  but  rather 
that  balls  of  matter  were  left  behind.  When  the  material  at  the 
equator  was  unusually  homogeneous,  a  ring  similar  to  Saturn's 
might  be  formed. 

Laplace  supposed  that  the  outermost  planet  was  formed  first,  but 
it  is  now  believed  that  several  of  the  planets  may  have  been  liberated 
at  about  the  same  time. 

Faye,  a  French  astronomer,  has  shown  that  the  inner  planets 
may  have  been  formed  before  the  outer  ones. 

The  retrograde  motions  of  the  satellites  of  Neptune  and  Uranus 
contradict  Laplace's  supposition  that  the  rings  from  which  they  were 
formed  rotated  as  if  solid,  before  they  broke  up  to  form  the  satellites. 
But  if  it  is  admitted  that  different  portions  of  the  ring  were  of  differ- 
ent degrees  of  density,  and  that  the  inner  edge  rotated  more  swiftly 
than  the  outer,  mathematicians  find  no  difficulty  in  accounting  for 
the  retrograde  motions  of  the  satellites.1 

According  to  Laplace's  theory  alone  the  inner  satellite  of  Mars 
should  not  complete  a  revolution  in  less  time  than  that  planet  re- 
quires to  rotate  on  its  axis.  This  anomaly  has  been  explained  in  a 
marvellous  manner  as  a  result  of  the  tides  which  the  sun  causes 
on  Mars.2 

397.  The  Evolution  of  Double  Stars.  —  Laplace's  theory  of  the  aban- 
.  donment  of  rings,  which  gradually  condensed  into  satellites,  answers 

very  well  for  the  solar  system,  but  fails  for  the  double  stars.     In  the 
solar  system  we  have  a  number  of  comparatively  small  planets  re- 

1  See  Young's  General  Astronomy,  Art.  914. 

2  An  elucidation  of  this  matter  in  a  popular  way  is  found  in  Ball's  Story  of  the 
Heavens,  Chapter  XXVII. 


268  DESCRIPTIVE    ASTRONOMY. 

volving  about  a  central  body,  which  is  750  times  as  massive  as  all  its 
planetary  attendants  put  together.  But  the  two  bodies  composing 
a  double  star  are  more  nearly  equal  to  each  other.  If  the  original 
nebula  were  quite  homogeneous,  rings  might  be  formed  as  supposed 
by  Laplace.  But  as  there  were  probably  great  differences  in  density 
in  different  parts  of  the  parent  nebula,  the  densest  portions  would 
attract  to  themselves  the  surrounding  matter.  Under  such  con- 
ditions it  is  probable  that  the  rotating  and  contracting  nebula  would 
separate  into  two  or  more  portions. 

Dr.  See1  has  specially  emphasized  the  fact  that,  while  the  ring 
formation  is  ideally  possible,  the  nebula  would  be  more  likely  to 
separate  into  two  globular  masses.  Many  double  nebulae  are  known, 
which  seem  to  substantiate  this  theory  of  "  fission."  Probably  such 
a  double  nebula  will  condense  into  a  double  star  after  thousands  or 
millions  of  years  have  elapsed. 

398.  Testimony  of  the  Nebulae. — We  have  seen  that  the  nebulae 
are   aggregations   of  tenuous  matter,   ranging  from   the  vast  filmy 
irregular  nebulae  to  the  neat  round  compact  planetary  nebulae.     Be- 
tween these  two  extremes  there  seems  to  be  every  gradation  of  size 
and  brightness. 

The  great  nebula  of  Andromeda  and  those  which  are  distinctly 
spiral  give  the  impression  that  they  may  be  rotating.  The  globular 
bright  spots  found  in  some  of  the  larger  nebulae  look  as  if  they  were 
condensations  of  the  surrounding  matter. 

Planetary  nebulae  usually  have  a  brightening  at  the  centre,  and 
nebulous  stars  seem  to  be  approaching  the  end  of  the  process  of 
transformation  into  stars. 

The  nuclei  of  planetary  nebulae  and  stars  like  those  in  the  Trape- 
zium of  Orion  (Theta  Orionis)  exhibit  spectra  similar  to  those  of 
the  nebulae  in  which  they  are  involved.  Immediately  around  the 
stars  of  the  Trapezium  there  is  a  dark  place,  as  if  the  matter  once 
there  had  been  used  up  to  form  the  stars. 

399.  Testimony   from   the   Stars. — The  wonderful   associations  of 
nebulae  and  stars,  such  as  are  found  in  the  Pleiades  and  in  Orion, 
point  to  a  close  connection.     Some  of  these  stars  have  wisps  of  neb- 
ulous matter  clinging  to  them,  as  photography  has  shown.     Others, 

1  Dr.  T.  J.  J.  See,  Professor  in  the  University  of  Chicago,  who  has  worked  out  an. 
elaborate  theory  of  the  evolution  of  double  stars. 


THE    NEBULA.  269 

though  giving  the  ordinary  spectra  of  stars  (§  353),  have  quite  an 
extensive  nebulosity  connected  with  them.  The  Wolf-Rayet  stars 
give  bright  line  spectra,  and  one  class  of  the  nebulae  does.  One 
naturally  concludes  that  we  have  different  stages  of  a  process  of  con- 
densation, which  will  finally  lead  to  the  production  of  such  highly 
finished  orbs  as  Sirius,  or  the  sun. 

400.  Testimony  of  the  Earth  and  Moon. — The  deeper  we  go  into 
the  crust  of  the  earth,  the  warmer  we  find  it.     Volcanoes  give  abun- 
dant evidence  of  the  presence  of  intensely  heated   matter  in  the  in- 
terior of  the  earth.     The  granite  which  we  prize  so  highly  owes  its 
toughness  to   its   having  passed   through  primeval  fires.      Statuary 
marble  is  but  common  limestone  which  has  been  metamorphosed  by 
heat.     Mountain   chains  are  thought  to  have   been  formed  by  the 
wrinkling  of  the  earth's  crust,  as  it  contracted  in  the  process  of  cool- 
ing.    The  earth  and  the  sun  are  composed  of  the  same  substances  in 
large  part.     Were  the  former  heated  to  incandescence  it  would  give 
essentially  the  same  spectrum  as  the  latter. 

The  moon  bears  the  marks  of  its  igneous  origin,  written  in  large 
characters  over  its  face.  The  following  extract  is  taken  from 
Nasmyth  and  Carpenter's  book  on  the  Moon  :— 

"  We  trust  that  we,  on  our  part,  have  shown  that  the  study  of  the  moon 
may  be  a  benefit  not  merely  to  the  astronomer,  but  to  the  geologist,  for  we 
behold  in  it  a  mighty  '  medal  of  creation,'  doubtless  formed  of  the  same  ma- 
terial and  struck  with  the  same  die  that  moulded  our  earth ;  but  while  the 
dust  of  countless  ages  and  the  action  of  powerful  disintegrating  and  denud- 
ing elements  have  eroded  and  obliterated  the  earthly  impression,  the  super- 
scriptions on  the  lunar  surface  have  remained  with  their  pristine  clearness 
unsullied,  every  vestige  sharp  and  bright  as  when  it  left  the  Almighty 
Maker's  hands." 

401.  Testimony  from  the  Planetary  Systems.  — We  note  the  following 
harmonies  in  the  motions  and  densities  of  the  planets. 

I.  They  all  revolve  eastward  about  the  sun,  in  orbits  nearly  cir- 
cular, which  lie  approximately  in  the  same  plane. 

II.  They  rotate  eastward  on  their  axes  (except  probably  Uranus 
and  Neptune),  the  planes  of  their  equators  being  but  little  inclined  to 
those  of  their  orbits  (except  probably  that  of  Uranus). 

III.  The  satellites  revolve  in  the  same  direction  in  which  the 


270  DESCRIPTIVE    ASTRONOMY. 

planets  rotate,  their  orbit  planes  being  nearly  coincident  with  the 
equators  of  their  respective  planets. 

IV.  The  four  inner  major  planets  are  small  bodies  of  great 
density;  they  rotate  slowly,  as  far  as  is  known.  The  four  outer 
major  planets  are  great  bodies  of  small  density ;  they  rotate  swiftly, 
as  far  as  is  known. 

402.  Testimony  of  the  Sun.  —  The  accepted  theory  of  the  source 
of  the  tremendous  quantity  of  heat  continually  radiated  by  the  sun 
is  the  contraction  theory   (§  86).      If  the  sun  be  now  contracting 
it  must  have  been  larger  1,000  years  ago  than  to-day.      Reasoning 
backward,  we  find  it  highly  probable  that  at  one  time  the  diameter 
of  the  sun  equalled  that  of  the  orbit  of  Mercury.     But  we  may  go 
yet  farther  back  in  imagination  and  see  the  sun  as  a  tenuous  nebu- 
lous mass,  the  confines  of  which  lie  beyond  the  orbit  of  the  farthest 
planet. 

403.  Is  the  Testimony  Sufficient?  —  The  human  mind  is  irresistibly 
attracted  toward  a  grand  and  far-reaching  theory,  which  explains  a 
variety  of   observed    results   by  a  single  process   of  development. 
With  a  limitless  duration  of  time  and  an  infinite  extent  of  space  at 
its  disposal,  it  leaps  over  the  most  stupendous  chasms  in  knowledge 
as  nimbly  as  a  mountain  goat  leaps  from  rock  to  rock,  scaling  the 
precipitous  heights  of  its  native  wilds. 

The  limitations  of  our  knowledge  are  so  great  that  the  Nebular 
Hypothesis  must  probably  remain  a  mere  theory,  as  long  as  man  in- 
habits the  earth ;  Bacon  has  said  that  the  subtlety  of  nature  tran- 
scends in  many  ways  the  subtlety  of  the  intellect  and  senses  of  man. 
Yet  the  theory  explains  many  facts  of  observation  so  simply  and  so 
reasonably,  that  the  speculations  of  men  will  probably  be  guided  by 
its  broad  lines  for  centuries  to  come. 

So  inadequate  is  the  sum  total  of  our  present  knowledge  of  the 
processes  of  celestial  evolution  that  we  are  led  to  cry  out  with  Job  : 
"  Lo,  these  are  parts  of  His  ways :  but  how  little  a  portion  is  heard 
of  Him?  but  the  thunder  of  His  power  who  can  understand?  " 

404.  The  Future  of  the  Visible  Universe.  —  As  the  sun  is  continually 
radiating  its  heat  away,  with  boundless  prodigality,  it  is  reasonable 
to  suppose  that  the  stars,  which  are  but  distant  suns,  are  doing  like- 
wise.    We  know  of  no  way  in  which  this  expenditure  is  to  be  repaid. 
We  can  look  forward  to  the  time  when  the  sun  will  become  a  cold 


THE    NEBULA.  271 

cinder,  feeling  its  way  by  the  starlight  through  the  darkness  of  infi- 
nite space.  But  will  there  be  starlight  then?  Many  of  the  stars  are 
larger  and  hotter  than  the  sun,  and,  though  much  diminished  in  radi- 
ance, will  yet  be  able  to  shed  a  kindly  though  feeble  light  upon  his 
pathway.  But  the  time  will  come  when  even  the  brightest  and  hot- 
test, having  radiated  its  heat  away,  will  roll  a  cold  corse  among  its 
dead  compeers.  Such  is  the  gloomy  teaching  of  our  philosophy. 

Once  there  lived  a  race  of  ephemerans,  whose  dwelling  place  was 
upon  a  thermometer.  The  span  of  life  of  one  of  them  was  but  a 
second.  Being  of  a  scientific  turn  of  mind  they  made  records  of  the 
readings  of  the  instrument.  After  observations  had  been  made  for 
ten  generations  they  promulgated  the  theory  that  the  mercury  was 
rising  one  hundredth  of  a  degree  every  second.  After  the  lapse  of 
ten  generations  more  the  theory  was  confirmed,  and  was  then  called 
a  law.  When  one  hundred  generations  had  passed  away,  the  law 
was  considered  so  firmly  established,  that  no  reasonable  ephemeran 
could  doubt  it.  It  was  the  one  grand  and  inexorable  law  of  nature : 
one  might  question  everything  else,  but  never  this.  During  the  next 
ten  generations  they  executed  a  laborious  triangulation,  determin- 
ing the  distance  over  which  the  mercury  must  still  travel  before  it 
reached  the  top  of  the  thermometer,  and  burst  the  glass  tube.  Then 
it  was  an  easy  matter  to  calculate  that  the  utter  ruin  of  their  beauti- 
ful dwelling  place  could  not  be  delayed  beyond  the  ten-thousandth 
generation. 

Great  was  the  humiliation  of  their  scientists,  but  still  greater  the 
joy  of  the  ephemerans  at  large,  when  it  was  found,  after  the  lapse  of 
two  thousand  generations,  that  the  mercury  was  actually  going  the 
other  way.  Even  the  scientists  were  constrained  to  admit  that  there 
were  more  things  in  heaven  and  earth  than  were  dreamed  of  in  their 
philosophy. 


272 


DESCRIPTIVE    ASTRONOMY. 


CHAPTER  XIV. 


THE   CONSTELLATIONS   IN    DETAIL. 

"  Sit,  Jessica.     Look  how  the  floor  of  heaven 
Is  thick  inlaid  with  patines  of  bright  gold : 
There  's  not  the  smallest  orb  which  thou  behold'st 
But  in  his  motion  like  an  angel  sings, 
Still  quiring  to  the  young-eyed  cherubins." 

SHAKESPEARE. 

405.  The  Greek  Alphabet.  —  Since  very  many  of  the  stars  are 
named  by  means  of  Greek  letters,  the  Greek  alphabet  is  subjoined. 
In  pronouncing  the  names  of  the  letters  e  should  be  given  like  ay 
in  bay.  I  is  pronounced  like  ee. 


a      Alpha. 
fi      Beta. 

Gamma. 

Delta. 

Epsilon. 

Zeta. 
T]       Eta. 

0  Theta. 

1  lo'ta. 

K      Kappa. 
X      Lambda. 
«,      Mu. 


v  NU. 

$  XI  (Ksee). 

o  Omicron. 

7T  PI. 

p  Rho. 

cr  Sigma. 

T  Tau. 

v  Upsilon. 

</>  Phi. 

X  Chi. 

$  PsI. 

to  Ome'ga. 


406.  Use  of  the  Data  in  this  Chapter.  —  Before  making  use  of  the 
data  in  this  chapter,  the  student  should  be  familiar  with  §§  I  and  8-12. 

Under  each  constellation  the  directions  for  finding  its  principal 
stars  are  first  given.  These  directions  presuppose  an  acquaint- 
ance with  Ursa  Major,  Ursa  Minor,  and  Cassiopeia  (§  10).  They 
should  be  used  in  conjunction  with  the  maps. 

When  attention  has  been  called  to  the  configuration  of  the  princi- 
pal stars  of  the  constellation,  the  chief  objects  of  interest  in  it  are 


THE  CONSTELLATIONS  IN  DETAIL.  273 

mentioned.  The  numbers  in  [  ]  refer  to  the  lists  at  the  end  of  the 
chapter.  For  the  mythological  history  of  the  constellations,  the 
reader  is  referred  to  a  classical  dictionary. 

At  the  end  of  the  chapter  the  right  ascensions  and  declinations 
of  interesting  telescopic  objects  are  given,  together  with  simple 
directions  for  finding  them  by  means  of  a  telescope  equatorially 
mounted,  and  provided  with  graduated  circles. 

407.  Andromeda,  the  Chained  Lady.     [Map  L]  —  Andromeda  may 
be  easily  learned  after  Cassiopeia.     A   line   drawn  from  Polaris  to 
ft  Cassiopeiae,  and  prolonged  an  equal  distance,  strikes  a,  which  is  the 
head  of  Andromeda :   it  is  also  one  corner  of  the  square  of  Pegasus. 
A  line  drawn  from  Polaris  to  e  Cassiopeiae,  and  prolonged  nearly  the 
same  distance  beyond,  ends  very  near  7,  which  is  in  one  foot  of  the 
figure.     The  row  of  stars  from  a  to  7  bounds  the  left  side  of  An- 
dromeda, so  that  her  body  lies  between  this  row  and  Cassiopeia. 
Her  outstretched  arms  run  from  X  to  77. 

The  great  nebula  [83]  which  is  plainly  visible  to  the  naked  eye 
is  in  line  with  /3  and  /a,  ^  being  half  way  between  the  other  two. 

It  has  been  described  in  §  389.  In  a  small  telescope  it  is  simply 
a  bright  oval  mass,  none  of  the  wonderful  details  of  its  structure 
being  perceptible. 

7  [6]  is  a  fine  double,  the  larger  star  being  orange  and  the 
smaller  sea-green :  the  small  star  is  a  very  close  double ;  7  is  there- 
fore really  a  triple  star.  The  small  star  is  of  the  sixth  magnitude, 
and  u"  distant. 

408.  Aquarius,  the  Water  Bearer.     [Map  V.]  —  A  line  drawn  from 
ft  Pegasi  to  a  of  the  same  constellation,  and  prolonged  as  far  again, 
terminates  just  east  of  a  group  of  fourth  magnitude  stars  having  the 
form  of  a  Y.     This  is   the  jar   from  which  Aquarius  pours  a  never 
exhausted   stream    of  water  which    meanders    southward    into    the 
mouth  of  the  Southern  Fish. 

The  remainder  of  this  dull-looking  constellation  lies  south  of  the 
jar,  and  extends  to  quite  a  distance  east  and  west  of  it.  Thirty 
degrees  south  and  a  little  east  of  the  Y  is  the  first  magnitude  star 
Fomalhaut,  in  the  Southern  Fish.  Between  the  two  lies  a  portion 
of  Aquarius  which  has  been  likened  to  the  contour  of  South  Amer- 
ica. It  is  formed  by  the  stars  0,  X,  r,  £,  c2,  and  i.  The  stream  from 
the  water  jar  to  the  Southern  Fish  is  marked  by  pretty  groups  of 

18 


274  DESCRIPTIVE    ASTRONOMY. 

stars,  and  is  indicated  on  the  map  by  a  dotted  line.  The  stars  a,  /?, 
v,  e,  and  3,  in  the  western  portion  of  the  constellation,  form  a  rude 
short-handled  dipper. 

f  [81],  the  central  star  of  the  Y  is  a  fine  binary,  the  components 
being  nearly  equal ;  they  are  3"  apart  now. 

A  little  over  a  degree  west  of  v  lies  a  small  bright  planetary  neb- 
ula [136],  with  a  stellar  nucleus:  it  is  of  a  greenish  blue  cast. 

Almost  directly  north  of  fi,  at  a  distance  of  5°,  lies  an  exceedingly 
compact  globular  cluster  [138]  of  faint  stars. 

409.  Aquila,  the  Eagle.     [Map  V.]  —  Altair,  the  brightest  star,  is 
easily  found  by  means  of  /3  and  7,  which  lie  on  either  side  of  it ;  the 
three  stars   lie  athwart  the  Milky  Way,  there  being  no  other  very 
bright  stars  in  the  immediate  vicinity.     The  triangle  formed  by  7, 
0,  and  X   embraces   most   of  the   bright   stars  of  the  constellation, 
which  bears  no  resemblance  to  an  eagle. 

A  degree  and  a  half  northeast  of  7  lies  the  sixth  magnitude  star 
TT  [69],  which  is  a  close  double,  a  test  for  the  power  of  a  three-  or 
four-inch  telescope,  on  a  fine  night;  the  components  are  nearly 
equal,  and  only  1^.5  apart. 

Three  degrees  east  and  a  trifle  south  of  12  Aquilae  lies  a  fan- 
shaped  cluster  [131]  of  telescopic  stars,  rj  varies  between  the 
fourth  and  fifth  magnitudes  in  a  period  of  seven  days  and  a  fraction. 

410.  Argo  Navis,  the  Ship.      [Map  III.]  —  Only  a  portion  of  this 
huge  constellation  is  visible  in  the  United  States.     The  rest  is  too 
far  south.     The  few  bright  stars  visible  to  us  lie  east  and  south  of 
Canis  Major,  and  can  be  identified  by  the  use  of  the  map,  after  that 
constellation  is  known. 

A  line  from  Sirius  to  7  Canis  Majoris,  when  prolonged  nearly 
twice  as  far,  terminates  just  north  of  a  diffuse  cluster  [102]  of  stars, 
some  of  which  are  visible  to  the  naked  eye. 

A  little  over  a  degree  east  of  the  preceding  and  20'  south  of  it  is 
a  circular  telescopic  cluster  [103]  30'  in  diameter.  A  degree  west 
of  [102]  lies  a  red  star  [150]  of  the  sixth  magnitude. 

411.  Aries,  the  Ram.     [Map  II.] — A  line  from  Polaris  through 
e  Cassiopeiae  to  7  Andromedse,  when  prolonged  a  distance  equal 
to   that   between    the    latter   stars,  pierces   the  triangle    composed 
of  a,    /3,  and  7    Arietis,    which  is  the   distinguishing   mark  of  the 
constellation.     The  triangle  is  in  the  Ram's  head.     His  body  lies 


THE  CONSTELLATIONS  IN  DETAIL.  275 

to  the  eastward,  and  bears  no  resemblance  to  the  configuration  of 
the  stars. 

7  [4]  is  a  fine  double  star,  the  components  of  which  are  nearly 
equal ;  the  distance  between  them  is  9". 

412.  Auriga,  the  Charioteer.     [Map  I.]  —  Capella,  the  brightest  star, 
is  of  the  first  magnitude,  and  forms  a  rude  square  with  Polaris,  e  Cas- 
siopeiae,  and  o  Ursae  Majoris,  the  star  in  the  nose  of  the  Great  Bear. 
It  is  one  of  the  brightest  stars  in  the   sky,   and  shines  with  a  pure 
white  light,     ft,  6,  L,  and  ft  Tauri  form  with  it  an  irregular  five-sided 
figure,  which  is  readily  discerned.     8  is  in  the  head  of  the  Charioteer : 
L  and  ft  Tauri  mark  his  feet.     He  carries  in  his  arms  a  kid  marked 
by  the  stars  e,  f,  and  77.  !  / 

14  Aurigae  [14]  is  a  triple  star  having  a  seventh  magnitude  com- 
panion at  a  distance  of  14"  and  one  of  the  eleventh  magnitude  at 
a  distance  of  13". 

Inside  of  the  triangle  formed  by  X,  t,  and  %  are  a  number  of  star 
clusters  most  of  which  lie  near  the  line  between  X  and  %.  Halfway 
between  X  and  L  is  a  rich  field  of  stars  [90]  fainter  than  the  seventh 
magnitude.  A  line  from  ft  to  a  point  midway  between  0  and  z>,  pro- 
longed as  far  again,  strikes  a  beautiful  cluster  [96]  of  small  stars : 
the  whole  field  seems  strewn  with  gold  dust.  These  stars  are  so 
closely  associated  that  one  must  believe  them  to  be  really  near  to- 
gether, and  not  merely  in  the  same  line  of  vision.  This  combina- 
tion of  stars  of  very  different  degrees  of  brightness  is  an  evidence 
that  a  faint  star  is  not  necessarily  at  a  great  distance  from  us.  A 
line  from  t  to  0,  prolonged  half  its  length,  ends  near  a  deep  red 
star  [149]  of  the  sixth  magnitude. 

413.  Bootes,  the  Bear  Keeper.     [Maps  I.  and  IV.]  —  A  line  from 
8  Ursae  Majoris  to  77  of  the  same  constellation,  prolonged  an  equal 
distance,  strikes  a  very  small  triangle  composed  of  the  fourth  mag- 
nitude stars  0,  L,  and  K,  which  are  in  the   uplifted  hand  of  Bootes. 
They  lie  midway  between  Polaris  and  Arcturus,  the  most  brilliant 
star  in  Bootes,  and  one  of  the  brightest  in  the  heavens:   it  has  a 
pronounced  ruddy  hue.     It  is  at  the  lower  end  of  an  immense  kite- 
shaped  figure  formed  by  ft,  8,  e,  a,  p,  and  7.     ft  is  in  the  head  of 
Bootes ;   7  and  5  are  in  his  shoulders ;  p  and  e  form  his  belt.     In  his 
right  foot  is  the  triangle  ?,  o,  TT;    in  his  left  foot  is  another  triangle, 
77,  T,  v.     Arcturus  is  in  his  sword. 


276  DESCRIPTIVE    ASTRONOMY. 

e  [41]  is  a  fine  slow  binary :  the  companion  is  of  the  sixth  magni- 
tude, and  3"  away.  The  large  star  is  yellow,  the  small  one  blue. 

f  [42]  is  a  fine  binary,  having  a  period  of  about  130  years.  The 
companion  is  now  less  than  4"  distant,  of  the  seventh  magnitude, 
and  purple.  The  distance  is  diminishing. 

1  [39]  is  5°  east  and  2°  north  of  77  Ursae  Majoris,  and  has  a 
companion  of  the  eighth  magnitude,  38"  away. 

TT  [40]  is  6°  east  and  3°  south  of  Arcturus,  and  has  a  companion 
of  the  sixth  magnitude,  6"  distant. 

414.  Camelopardus,  the  Camelopard.     [Map  I.]  — The  stars  in  this 
constellation  are  faint.     The  head  of  the  creature  consists  of  four 
fifth  magnitude  stars  situated  one  fifth  of  the  way  from  Polaris  to 
the  bowl  of  the  Great  Dipper.     His  fore  feet  are   almost  on  the 
head  of  Auriga,  while  his  hind  feet  are  in  position  to  give  Perseus 
a  kick  in  the  stomach.     If  he  were  not  such  a  weakling,  he  might 
give  trouble. 

There  is  a  rich,  though  coarse  cluster  [89]  two  thirds  of  the 
way  from  a  Persei  to  &  Aurigae.  It  is  close  to  the  fifth  magnitude 
star  7  Camelopardi. 

415.  Cancer,  the  Crab.     [Maps  III.  and  I.] — The  principal  start 
form  an   inverted  Y,  as  shown  on   Map   III.     A  line   drawn  from 
Polaris  to  h  Ursae  Majoris,  a  fifth  magnitude  star  in  the  head   of 
the  Great  Bear,  and  prolonged  i^  times  its  own  length,  strikes  i,  the 
uppermost  star  in  the  A. 

f  [27]  is  found  by  alignment  with  Castor  and  Pollux.  It  is  a 
fine  multiple  star,  and  has  been  described  in  §  372.  The  two  stars 
forming  the  bright  one  are  in  rather  rapid  motion,  sixty  years 
sufficing  for  a  revolution.  The  visible  companion  of  this  binary  is 
5"  distant.  The  large  star  looks  oblong  with  a  high  power  on  a 
three-  or  four-inch  telescope. 

Between  7  and  &  lies  the  cluster  [104]  Praesepe,  the  Beehive, 
which  is  visible  to  the  naked  eye  on  a  moonless  night.  A  good- 
sized  opera-glass  shows  it  better  than  a  larger  telescope. 

416.  Canes  Venatici,   the  Hunting  Dogs.     [Map  I.] — This  con- 
stellation is  not  especially  noteworthy.     Its  brightest  star,  a  or  12, 
is  called  Cor  Caroli  (the  Heart  of  Charles  II.  of  England),  and  is 
found  by  prolonging  a  line  from  Polaris  to  e  Ursae  Majoris  half  its 
length. 


THE  CONSTELLATIONS  IN  DETAIL.  277 

Cor  Caroli  [36]  has  a  sixth  magnitude  companion  20"  distant. 

2  [33]  is  an  orange  star  of  the  fifth  magnitude,  having  a  blue 
companion  of  the  ninth  magnitude,  n"  distant. 

There  is  a  bright  globular  cluster  [114]  containing  upwards  of 
1,000  stars,  lying  nearly  midway  between  Cor  Caroli  and  a  Bob'tis 
(Arcturus),  but  a  little  nearer  the  latter:  it  is  close  to  a  star  of  the 
sixth  magnitude. 

The  Great  Spiral  Nebula  [113]  (§  391)  lies  about  one  fourth  of 
the  way  from  77  Ursae  Majoris  to  Cor  Caroli.  It  is  called  the  Whirl- 
pool Nebula,  but  in  small  telescopes  it  looks  simply  like  a  faint 
double  nebula.  The  entire  constellation  is  plentifully  besprinkled 
with  faint  nebulae. 

417.  Canis  Major,  the  Great  Dog.     [Map  III.] — This  constella- 
tion  is   best  learned   after   Orion.     The   line   of  the  three  stars  in 
Orion's   belt   prolonged    eastward    passes   near    Sirius,  which    is  a 
in   this  constellation,   and    by  far   the    brightest  fixed    star   in   the 
heavens.     The  triangle  S,  e,  77  is  in  the  haunches  of  the  animal,  and 
.Sirius  is  in  his  head :   ft  is  the  extremity  of  one  uplifted  fore  paw. 
The  animal    sits    upright,   in    the    attitude   of  begging   his    master 
Orion  for  permission  to  put  his  teeth  into  the  Hare,  which  is  under 
Orion's  feet. 

Sirius  is  a  very  interesting  double  star  (§  370),  but  is  much  too 
difficult  for  a  small  telescope,  the  faint  companion  being  in  the 
blaze  of  light  surrounding  the  bright  star.  The  period  of  revolution 
is  about  fifty  years. 

fji  [24]  has  a  ninth  magnitude  companion  at  a  distance  of  less 
than  4". 

A  superb  cluster  [100]  visible  to  the  naked  eye  lies  about  one 
third  of  the  way  from  Sirius  to  e.  There  is  a  ruddy  star  near  the 
centre :  many  of  the  brighter  stars  are  arranged  in  curves. 

418.  Canis  Minor,  the  Little  Dog.     [Map  III.]  —  It  is  well  to  learn 
Gemini  before  Canis  Minor.     A  line  from  Polaris  to  13  Geminorum 
(Pollux),  prolonged  one  third  as  far  again,  reaches  Procyon,  a  first 
magnitude  star,  which  is   a  Canis   Minoris.     The   only   other  con- 
spicuous star  is  /3,  which  is  4°  northwest  of  Procyon. 

Procyon  is  of  interest  because  of  its  irregular  proper  motion, 
supposed  to  be  caused  by  the  presence  of  close  companions,  which 
have  often  been  searched  for  by  the  largest  telescopes,  but  without 


278  DESCRIPTIVE    ASTRONOMY. 

success.  In  the  same  field  with  Procyon  is  a  star  of  the  seventh 
magnitude  which  is  a  close  double.  The  components  are  only  i".5 
apart,  but  the  star  can  be  elongated  by  a  good  four-inch  glass. 

419.  Capricornus,  tne  Goat.     [Map  V.]  —  It  is  well  to  know  Cyg- 
nus  before  attempting  to  learn  Capricornus.     A  line  from  Polaris 
to   7   Cygni    (where   the   arm  of  the  cross  is  fixed   to  the   upright 
piece),  prolonged  an  equal  distance,  reaches  the  naked-eye  double 
a,  below  which  is  ft  at  a  distance  of  2°. 5.     S  and  7  form  another 
such    pair    of    stars,    and    ^    and    co    a   third.     The    constellation 
i-s  chiefly  embraced   in   the   triangle  formed  by  these   three  pairs. 
a  and  ft  are  in   the   head  of  the  animal,  while  i/r  and  co  are  in  his 
knees ;   the  rest  of  the  Goat  may  be  supplied  to  suit  the  fancy. 

p  [73]  and  TT  [72]  are  pretty  doubles,  each  having  a  companion 
of  the  ninth  magnitude,  less  than  4."  away.  A  good  night  is  needed 
for  their  observation  with  a  four-inch  glass. 

420.  Cassiopeia,  the  Lady  in  the  Chair.     [Map  I.]  —  This  brilliant 
constellation   is  quickly  found  by  using  Map  I.   according  to  the 
directions  in  §  10.    The  stars  e,  8,  7,  a,  ft,  and  K  form  a  rude  broken- 
backed  chair.     The  Lady,  however,  refuses  to  sit  in  it,  preferring 
to  sit  on  empty  space.     The  stars  ft,  a,  7,  and  K  form  her  body; 
8  is   in  her  knee,  and  i  in  her  foot;   £  is   in   her  head;   her  arms 
are  uplifted,   possibly   in  prayer  to   the   gods   to  spare  her   lovely 
daughter  Andromeda,  who  has  been  chained  to  a  rock,  as  prey  for 
a  sea  monster. 

77  [i]  is  a  splendid  binary,  having  a  purple  companion  of  the 
eighth  magnitude,  5"  distant.  It  is  less  than  half  the  way  from 
a  to  7.  The  period  of  revolution  is  200  years :  the  combined  mass 
of  the  two  stars  is  thought  to  be  from  five  to  ten  times  that  of  the 
s-un.  Near  tc  appeared  Tycho's  new  star  described  in  §  375. 

As  the  Milky  Way  runs  through  Cassiopeia,  there  are  many 
beautiful  fields  which  can  be  best  seen  with  a  low  power. 

Near  ft,  between  p  and  a,  is  a  large  cloud  of  minute  stars 
[142]  discovered  by  Caroline  Herschel,  the  sister  of  Sir  William 
Herschel. 

Between  TT  and  o  in  the  uplifted  left  hand  of  the  Lady  is  a 
magnificent  region. 

One  degree  east  and  a  little  north  of  8  is  a  beautiful  field  [84]. 
A  line  from  B  to  a,  prolonged  i-J-  times  its  former  length,  ends  near 


THE  CONSTELLATIONS  IN  DETAIL.          279 

R,  a  vivid  red  star  which  varies  from  the  fifth  to  the  twelfth  magni- 
tude in  a  period  of  433  days. 

421.  Centaurus,  the  Centaur.    [Maps  III.  and  IV.]  —  Centaurus,  even 
when  most  favorably  situated,  is  too  near  the  southern  horizon  for 
satisfactory  observation  in  the  United  States,  except  in  Florida  and 
Southern  Texas.     It  is  of  especial  interest,  because  it  contains  our 
nearest  neighbor  among  the  stars,  a  Centauri. 

422.  Cepheus.     [Map  I.] — This  constellation  lies  between  Cassi- 
opeia and  Draco.     Cepheus  is  the  husband  of  Cassiopeia,  who,  with 
her  daughter  Andromeda,  nearly  monopolizes  the  brilliancy  of  the 
family.     The  five  brightest  stars  are  a,  ft,  7,  t,  and  f,  which  form  a 
figure  composed  of  a  rude  square  surmounted  by  a  triangle  which 
is  nearly  isosceles,     a  forms  with  Polaris  and  7  Cassiopeiae  an  isos- 
celes triangle  which  is  nearly  equilateral.     Near  f  are  8  and  e :   the 
three  are  in  the  head  of  the  figure. 

ft  [77]  is  a  double  star,  the  companion  being  blue,  of  the  eighth 
magnitude,  and  14"  distant. 

£  [82]  has  a  companion  of  the  seventh  magnitude,  41"  distant: 
the  primary  is  yellow,  the  companion  blue  :  the  main  star  is  a  noted 
variable,  having  a  period  of  5^  days. 

f   [80]  has  a  blue  seventh  magnitude  companion,  6"  distant. 

423.  Cetus,  the  Whale.    [Maps  II.  and  V.]  —A  line  from  Polaris  to 
8  Cassiopeiae,  prolonged  so  that  the  prolongation  is  2\  times  the  ori- 
ginal length  of  the  line,  reaches  the  centre  of  this  huge  and  ungainly 
constellation,  which  can  be  best  learned  by  following  the  dotted  lines 
given  on  the  map.     The  monster  has  about  the  shape  of  a  walrus. 
The  most  noticeable  portion  of  the  constellation  is  an  irregular  pen- 
tagon, rudely  kite-shaped,  formed  from  the  third  magnitude  stars  /3, 77, 
01,  f,  and  r.     The  pentagon  formed  by  a,  7,  f 2,   /x,  and  X  marks  the 
head. 

Nearly  half  way  from  7  to  flies  o  (Mira)  [143],  the  wonderful 
variable  described  in  §  376.  It  is  visible  to  the  naked  eye  only  six 
weeks  in  the  year. 

7  [8]  is  not  a  very  difficult  double  for  a  four-inch  glass.  A  star 
of  the  seventh  magnitude  nestles  close  to  the  larger  star :  the  dis- 
tance is  2". 5. 

a  [144]  is  a  fine  orange-colored  star,  having  a  blue  neighbor  in 
the  same  low-power  field. 


280  DESCRIPTIVE    ASTRONOMY. 

424.  Columba,  the  Dove.    [Map  II.]— The  full  name  is  Columba 
Noachi  or  Noah's  Dove.     The  asterism  lies  south  of  Lepus,  and  is 
too  low  down  in  the  south  to  be  seen  well.     A  line  drawn  from  /3 
Orionis  (Rigel)  to  fi  Leporis,  and  prolonged  as  far  again,  terminates 
near  a  and  {3,  the  two  brightest  stars. 

425.  Coma  Berenices,  the  Hair  of  Berenice.     [Map  I.] — This  con- 
stellation consists  of  faint  stars ;   most  of  those  visible  to  the  naked 
eye  are  of  the  fifth  and  sixth  magnitudes.     They  are  well  crowded 
together.     A  line  from  Polaris  to  8  Ursae  Majoris,  when  prolonged  an 
equal  distance,  terminates  near  the  most  crowded  part  of  the  asterism. 
It  is  a  fine  sight  in  a  small  opera-glass. 

A  little  over  one  third  of  the  way  from  ??  Bootis  (near  Arcturus) 
to  0  Leonis  is  the  fifth  magnitude  star  42  ;  50'  northeast  of  this  is  a 
condensed  mass  of  minute  stars  [112],  which  cannot  be  well  seen 
with  a  telescope  of  less  than  four  inches  aperture. 

426.  Corona  Borealis,  the  Northern  Crown.     [Map  I.]  —  Corona  lies 
a  little  south  of  a  line   from  a  Bootis  (Arcturus)  to  a  Lyrse  (Vega)r 
at  about  one  third  the  distance  from  the  former  to  the  latter.     Seven 
of  its  principal  stars  form  a  figure  so  similar  to  a  crown  that  it  is  in- 
stantly recognized. 

f  [44]  has  a  bluish  green  companion  of  the  sixth  magnitude  6'r 
distant. 

A  degree  south  of  e  is  a  ninth  magnitude  star  called  T  Coronae 
[155]*  It  suddenly  blazed  up  in  May,  1866,  and  equalled  a  in 
brightness ;  it  then  slowly  declined,  and  after  a  month  reached  its 
former  low  estate,  which  it  has  held  ever  since. 

427.  Corvus,  the  Crow.     [Map  IV.]  — A  line  from  Polaris  to  B  Ursae 
Majoris,  prolonged  until  it  is  3^  times   its   former  length,  strikes  a 
snjall  but  conspicuous  quadrilateral,  15°  west  and  10°  south  of  aVir- 
ginis  (Spica).     a  is  in  the  Crow's  bill;   the  Crow  stands  upon  and 
pecks  at  Hydra. 

8  [34]  is  accompanied  by  a  purple  star  of  the  eighth  magnitude, 
at  a  distance  of  24". 

428.  Crater,  the  Cup.    [Map  III.]  —  Crater  adjoins  Corvus  on  the 
west,  and  stands  upon  Hydra.     The  stars  77,  f,  7,  S,  e,  and  6  form  the 
bowl  of  a  crooked  goblet,  in  the  base  of  which  are  a  and  /3.    The  gob- 
let leans  as  if  to  discharge  its  contents  upon  its  neighbor,  the  Crow. 

Just  east  of  a  and  in  the  same  field  of  view  with  a  very  low  power 


THE    CONSTELLATIONS    IN 

is  R  [152],  a  notable  red  star  of  the  eighth  magnitude.  Sir  William 
Herschel  described  it  as  "  scarlet,  almost  blood-colored ;  a  most 
intense  and  curious  color." 

429.  Cygnus,  the  Swan.     [Map  I.]  —  Cygnus  is  readily  discovered 
by  following  the  directions  for  using  Map  I.  given  in  §  10.     It  lies  in 
the  Milky  Way,  just  east  of  Lyra,  and  is  quickly  recognized  by  the 
cross,  the  upright  piece  of  which  is  composed  of  a,  7,  77,  ^,  and  /8, 
and  has  the  same  trend  as  the  Milky  Way.     The  cross  arm  consists 
of  the  stars  8,  7,  and  e. 

ft  is  in  the  Swan's  head,  and  a  in  its  tail.  The  cross  piece  of  the 
cross,  extended,  forms  the  wings  of  the  bird. 

ft  [66]  has  a  blue  seventh  magnitude  companion  at  a  distance 
of  34".  It  is  the  finest  colored  double  for  a  small  telescope  in  the 
northern  sky ;  the  colors  are  beautifully  seen  by  putting  the  telescope 
slightly  out  of  focus. 

fi  [78]  is  a  much  closer  double,  the  fifth  magnitude  companion 
being  only  4"  distant.  A  third  star  of  the  seventh  magnitude  is 
over  200''  distant. 

17  [68]  lies  in  a  beautiful  field,  and  has  a  ninth  magnitude  com- 
panion 26"  distant. 

61  [76],  which  is  in  one  corner  of  a  parallelogram  formed  by  a, 
7,  e,  and  itself,  is  a  pretty  double  when  seen  with  a  low  power :  the 
components  are  nearly  equal.  This  star  is  celebrated  as  the  first 
one  the  distance  of  which  from  us  was  measured.  It  is  about 
550,000  times  as  far  off  as  the  sun. 

There  are  fine  fields  in  many  places,  especially  within  a  few  de- 
grees of  a  (Deneb).  One  of  the  best  is  a  little  north  of  the  middle 
of  a  line  from  a  and  8,  near  o.  In  the  northeast  corner  of  the  con- 
stellation, about  half  way  between  p  and  Tr1,  is  a  large  cluster  [139] 
in  a  rich  vicinity. 

430.  Delphinus,  the  Dolphin.     [Map  V.]  — A  line  from  Polaris  to 
a  Cygni,  when  prolonged  until    it  is  two  thirds  longer  than  before, 
strikes  a  small  diamond,  composed  of  three   stars  of  the  fourth  mag- 
nitude and  one  of  the  third.     These,  with  a  fifth  of  the  fourth  mag- 
nitude, which  lies  southwest  of  them,  form   a  narrow  wedge,  called 
Job's  Coffin.     This  is  the  principal  portion  of  Delphinus. 

7  [74]  is  a  golden  yellow  star  having  a  greenish  blue  companion 
of  the  sixth  magnitude,  at  a  distance  of  u". 


282  DESCRIPTIVE    ASTRONOMY. 

431.  Draco,   the   Dragon.      [Map  I.] — The  head  of  the  Dragon 
consists  of  a  bright  quadrilateral  formed  of  ft,  7,  f ,  and  v,  which  is  so 
situated  as  to  form  an  equilateral  triangle  with  Cassiopeia  and  the 
bowl  of  the  Great  Dipper,  Polaris  being  inside  of  the  triangle.     It 
also  forms  a  much  smaller  right  triangle  with  a  Lyrae   (Vega)   and 
a  Cygni,  the  right  angle  being  at  Vega. 

From  the  head  the  constellation  winds  in  magnificent  convolu- 
tions, shown  by  the  dotted  line  on  the  map,  around  between  the  two 
Bears.  X,  the  last  bright  star  in  the  tail,  is  two  thirds  of  the  way 
from  Polaris  to  the  centre  of  the  bowl  of  the  Great  Dipper. 

About  half  way  between  £  Ursse  Majoris  (Mizar)  and  7  Ursae 
Minoris  (one  of  the  two  brighter  stars  in  the  Little  Dipper)  lies  a, 
which  is  distinguished  as  having  been  the  pole  star  four  or  five 
thousand  years  ago.  About  half  way  between  8  and  f  lies  the  pole 
of  the  ecliptic,  which  is  near  a  bright  planetary  nebula  [125],  35"  in 
diameter.  Unlike  most  such  objects,  it  can  be  seen  very  well  with  a 
four-inch  glass. 

fji  [52]  is  a  neat  double,  the  two  stars  being  nearly  equal  in  bright- 
ness, and  less  than  3"  apart.  A  planetary  nebula  has  been  men- 
tioned above. 

432.  Equuleus,  the  Little  Horse.     [Map  V.]  —  a  lies  7°  west  and 
nearly  5°  south  of  e  Pegasi,  which  is  in  the  nose  of  the  animal.     It 
contains  only  five  stars  above  the  sixth  magnitude. 

e  [75]  has  a  companion  of  the  seventh  magnitude  at  a  distance  of 
n" ':  the  main  star  is  a  close  rapid  binary,  which  now  looks  elon- 
gated in  a  four-inch  telescope,  armed  with  a  high  power. 

433.  Eridanus,  the  River.     [Map  II.] — Three  degrees  north  and 
two  west  of  /3  Orionis  lies  @  Eridani,  which  may  be   considered  as 
the  source  of  the  river.     Thence  it  flows  west,  following  the  sinuous 
line  on  the  map,  till  it  reaches  the  star  TT  Ceti,  where  it  laves  the 
paws  of  Cetus;   then  it  drops  south  about  5°,  thence  east,  southeast, 
and  southwest  in  succession,  till  it  is  lost  beneath  our  horizon. 

32  [n],  which  has  a  right  ascension  of  3  h.  49m.  and  a  south 
declination  of  3°  15',  is  a  fifth  magnitude  star  having  a  companion  of 
the  seventh  magnitude  7"  distant.  The  primary  has  been  called 
topaz-yellow,  and  the  companion  sea-green. 

434.  Gemini,  the  Twins.     [Maps  I.  and  II.] — A  line  drawn  from 
the  bowl  of  the  Little  Dipper  to  the  head  of  the  Great  Bear,  and 


THE  CONSTELLATIONS  IN  DETAIL.  283 

prolonged  an  equal  distance,  terminates  near  the  two  bright  stars  a 
and  /3  (Castor  and  Pollux).  Pollux  is  the  brighter  of  the  two. 
These  two  are  in  the  heads  of  the  twins,  who  stand  side  by  side. 
The  chief  stars  can  be  traced  by  the  dotted  lines  on  Maps  I.  and  II. 
The  entire  figure  is  much  like  an  end  view  of  an  upright  piano. 
a  and  /3  are  at  the  top,  p,  7,  and  f  at  the  bottom,  while  X  and  f  are 
at  the  key-board.  The  summer  solstice  is  close  to  the  fifth  magni- 
tude star  I,  which  is  a  little  west  and  north  of  rj  and  /JL. 

Castor  [26]  is  a  magnificent  double,  the  components  differing  one 
magnitude  in  brightness,  and  being  nearly  6"  apart.  It  is  a  binary, 
the  period  of  which  is  thought  to  be  about  1,000  years. 

8  [25]  has  an  eighth  magnitude  companion  at  a  distance  of  f. 

Four  degrees  west  and  two  north  of /x  (at  the  base  of  the  back  of 
the  piano)  is  a  cluster  [97],  visible  to  the  naked  eye  as  a  faint  cloud 
on  the  sky.  It  is  20'  in  diameter  and  consists  of  stars  from  the  ninth 
magnitude  down  to  the  faintest  points  of  light. 

435.  Hercules.     [Maps  I.  and  IV.] — Directly  east  of  Corona  lies 
the  belt  of  Hercules,  composed  of  the  stars  e  and  f ;   (3  and  8  are  in 
the  shoulders ;   ??  and  TT  are  in  the  thighs ;   a  marks  the  head.     The 
limbs  and  arms  are  traced  by  the  dotted  lines  on  the  maps.     The 
whole  forms   a  fair   picture    of  a   giant,  with    his  head  toward  the 
equator. 

a  [54]  is  a  fine  double,  having  an  emerald  companion  of  the  sixth 
magnitude  5"  away :  it  is  also  variable. 

p  [57]  is  a  binary,  having  a  greenish  companion  of  the  fifth  mag- 
nitude at  a  distance  of  4" :  it  is  near  TT  in  one  thigh. 

8  [55]  in  one  shoulder  has  an  eighth  magnitude  companion,  which 
has,  if  one  compares  the  estimates  of  different  observers,  nearly  all 
the  colors  of  the  rainbow,  and  is  at  a  distance  of  19". 

The  finest  globular  cluster  [118]  in  the  northern  hemisphere, 
pictured  in  Fig.  173,  is  one  third  of  the  way  from  77  to  £,  and  is  just 
visible  to  the  naked  eye.  The  stars  are  so  thickly  crowded  near 
the  centre,  that  a  small  telescope  shows  them  simply  as  a  neb- 
ulous mass. 

About  one  third  the  way  from  t,  in  one  foot,  to  rj,  in  the  opposite 
thigh,  is  a  very  condensed  cluster  [121],  which  is  fine,  but  inferior  in 
interest  to  the  preceding. 

436.  Hydra,  the  Snake.     [Maps  II.  and  III.]  —A  line  from  Polaris 


284  DESCRIPTIVE    ASTRONOMY. 

through  the  middle  of  the  triangle  which  forms  the  head  of  the 
Great  Bear,  carried  on  through  Cancer,  meets  the  head  of  Hydra, 
which  is  just  beyond  Cancer ;  the  head  is  a  good  representation  of 
that  of  a  hissing  snake.  Thence  it  may  be  traced  in  a  south  and 
east  direction  by  following  the  dotted  line  on  the  map.  A  line  from 
Polaris  through  h  at  the  vertex  of  the  obtuse  angle  of  the  triangle  in 
the  Great  Bear's  head,  passing  in  front  of  the  Sickle  in  Leo  (through 
K  Leonis)  meets  a,  which  is  also  called  Cor  Hydrse.  The  distance 
from  a  to  K  Leonis  is  one  half  the  distance  of  the  latter  from  Polaris. 
One  is  helped  in  tracing  the  eastern  end  of  the  constellation  by  the 
recognition  of  Corvus,  which  stands  upon  it. 

e  [28],  the  northernmost  star  in  the  head,  has  a  blue  companion 
of  the  eighth  magnitude  at  a  distance  of  3". 5. 

At  a  right  ascension  of  loh.  20  m.,  2°  south  of  ^,  is  a  bright  plan- 
etary nebula  [108],  which  appears  as  large  as  Jupiter  when  the  latter 
is  at  opposition. 

437.  Lacerta,  the  Lizard.     [Map  I.] — Lacerta  lies  between   Cyg- 
nus  and   Andromeda.      The   middle   point  of  a  line   connecting  a 
Cygni  with  a  Andromedae   lies  a  little  south  of  the   centre  of  the 
constellation. 

Two  and  a  half  degrees  west  of  7,  which  i-s  the  brightest  star  in 
the  constellation,  lies  a  fair  cluster  [141].  The  constellation  furnishes 
some  fine  fields,  when  viewed  with  a  low  power. 

438.  Leo,   the   Lion.      [Map   III.] — A  line   from   Polaris    to   the 
middle  point  of  a  line  connecting  a  Ursae  Majoris  and  k  of  the  same 
constellation,   when    prolonged    to    nearly   three    times    its  original 
length,   passes  through   a  conspicuous  figure  known  as  The  Sickle, 
and  terminates  at  a  (Regulus),  in  the   end  of  the    handle  of  the 
Sickle.     At  the  east  of  this  figure  is  a  conspicuous  right-angled  tri- 
angle which  lies  in  a  line  drawn  from  Polaris  through  the  bowl  of 
the  Great  Dipper.     The  Sickle  constitutes  the  head   and  the   fore 
part  of  the  body  of  the  crouching  lion.     The  large  triangle  is  in  his 
haunches.     Regulus  is  sometimes  called  The  Lion's  Heart. 

7  [30]  is  a  golden  yellow  star,  having  a  companion  of  the  fourth 
magnitude,  at  a  distance  of  3".  5.  It  is  one  of  the  finest  binaries  in 
the  northern  sky :  its  period  is  about  400  years. 

i  [32],  the  nearest  bright  star  south  of  the  west  end  of  the  right 
triangle,  has  a  bluish  companion  of  the  seventh  magnitude,  less  than 
3"  away. 


THE  CONSTELLATIONS  IN  DETAIL.  285 

A  little  over  half  the  way  from  a  to  f  is  the  crimson  variable  R 
[151],  which  ranges  between  the  fifth  and  tenth  magnitudes ;  the 
period  is  312  days. 

439.  Leo  Minor,  the  little  Lion.     [Map  I.]  — Adjoining  the  Sickle, 
in  a  line  from  it  to  the  bowl  of  the  Great  Dipper,  lies  Leo  Minor,  a 
shapeless   constellation    containing  a  few  naked-eye  stars,  three  of 
which  are  as  bright  as  the  fourth  magnitude. 

440.  Lepus,  the  Hare.    [Map  II.]  — Lepus  crouches  under  Orion's 
feet,  and  does  not  particularly  resemble  a  hare. 

7  [21]  is  a  triple  star;  the  larger  companion  is  of  the  seventh 
magnitude,  and  is  93"  distant;  the  small  companion  is  45"  from 
the  other  one,  and  is  visible  with  a  three-inch  glass. 

45  [J9]  is  a  seventh  magnitude  star  I  J°  east  of  a ;  it  has  four  com- 
panions visible  with  a  small  telescope,  at  distances  varying  from  60" 
to  126".  There  are  four  other  companions  to  be  seen  with  larger 
telescopes. 

A  line  from  a  to  p,  prolonged  two  thirds  of  its  length,  ends  close 
to  the  crimson  star  R  [147],  which  varies  from  the  sixth  to  the  ninth 
magnitude;  the  period  is  438  days. 

441.  Libra,  the  Scales.     [Map   IV.]  — Libra  is  best  learned   after 
Virgo  and   Scorpio,  between  which  it  lies,     a  lies  a  little  more  than 
half  way  from  a  Virginis  (  Spica )  to  /3   Scorpii.     The  chief  config- 
uration is  a  quadrilateral  formed  by  a,  fi,  7,  and  i. 

a  looks  elongated  to  a  keen  eye ;  an  opera-glass  shows  that  it 
has  a  fifth  magnitude  companion. 

B  [153]  is  a  variable,  situated  4°. 5  west  and  i°  north  of  /3.  Its 
period  is  2\  days,  and  it  varies  from  the  fifth  to  the  sixth  magnitude. 
The  change  in  brightness  consumes  12  hours. 

$  [154]  is  a  pale  green  star. 

442.  Lupus,  the  Wolf.      [Map  IV.] — Lupus  lies  south  of  Libra, 
and  even  when  best  seen  is  too  near  the  southern  horizon  for  observ- 
ers in  middle  north  latitudes. 

443.  Lynx,  the  Lynx.      [Map   I.] — The    Lynx    occupies   a  dull 
region  between  Ursa  Major  on  one  side,  and  Auriga  and  Gemini  on 
the  other.      The  leading   stars  form  an  irregular  line,  traced  on  the 
map. 

38  [29]  in  the  southeastern  corner  of  the  constellation,  has  a  lilac 
companion  of  the  seventh  magnitude,  3"  distant.  The  pair  38  and 


286  DESCRIPTIVE    ASTRONOMY. 

40  form  an  equilateral    triangle  with  two  pairs  in  the  feet  of  the 
Great  Bear. 

5  [148]  is  a  fiery  red  star  of  the  sixth  magnitude,  in  a  fine  group, 

444.  Lyra,  the  Harp.     [Map  I.]  — The  leader  of  this  constellatioa 
(Vega)  is  one  of  the  brightest  of  the  first  magnitude  stars.     To  the 
naked  eye  its  color  is  pale  sapphire.     It  is  easily  identified  by  means 
of  the  two  fourth  magnitude  stars,  e  and  f,  which  form  with  it  an 
equilateral  triangle,  each  side  of  which  is  nearly  2°  in  length.      The 
constellation  lies   between   Hercules   and   Cygnus.     The  equilateral 
triangle  is  perched  on  one  corner  of  a  rhomboid,  £  being  common 
to  both  figures. 

a  (Vega)  [60]  has  a  blue  companion  of  the  tenth  magnitude,  48" 
distant. 

/3  [63]  is  a  multiple  star,  having  three  companions  of  about  the 
eighth  magnitude,  at  distances  of  46",  66",  and  86",  respectively.  It 
is  also  one  of  the  noted  variables.  See  §  378. 

e  [61]  is  one  of  the  equilateral  triangle,  and  appears  elongated  to 
the  average  eye :  a  sharp  eye  splits  it  into  two  stars.  An  opera- 
glass  separates  them  widely,  and  a  small  telescope  shows  each  star 
as  a  double.  The  distance  between  the  components  of  one  pair  is 
3" ;  the  other  pair  is  a  little  closer. 

£  [62]  has  a  fifth  magnitude  companion,  44"  distant. 

8°  east  of  Vega  are  the  two  stars  rj  \6$]  and  0.  The  former 
has  a  blue  companion  of  the  ninth  magnitude,  28"  distant.  8,  one 
of  the  stars  of  the  rhomboid,  is  double  in  an  opera-glass,  and  is  sit- 
uated in  a  fine  field. 

Beautiful  fields  lie  between  e  and  R,  which  is  5°  northeast  of  it. 
The  only  annular  nebula  [132]  which  small  telescopes  reveal  lies  one 
third  of  the  way  from  ft  to  7.  It  has  been  described  in  §  391. 

445.  Monoceros,  the  Unicorn.     [Maps  III.  and  II.] — This  constel- 
lation contains  only  four   stars  as  bright  as  the  fourth  magnitude. 
It  lies   east  of  Orion,  and  stretches  itself  in  the  Milky  Way  between 
Canis  Major  and  Canis  Minor. 

8  [22]  lies  in  the  northwestern  part  of  the  constellation,  at  a  right 
ascension  of  6  h.  19  m.  A  line  from  X  Orionis  (in  his  head)  to  a 
Orionis,  prolonged  i|  times  its  own  length,  stops  just  south  of  8. 
It  is  a  golden  yellow  star  with  a  lilac  companion  of  the  seventh 
magnitude,  13"  distant:  it  is  in  a  splendid  field. 


THE  CONSTELLATIONS  IN  DETAIL.  287 

II  [23],  which  lies  in  the  southwestern  part  of  the  constellation, 
has  a  double  companion  of  the  sixth  magnitude,  7"  distant.  The 
components  of  the  companion  are  2". 3  apart.  It  is  a  star  of  the 
fourth  magnitude,  about  three  eighths  of  the  way  from  Sirius  to 
a  Orionis  (Betelgueuse),  a  little  east  of  a  direct  line. 

2°  east  of  8,  and  i°  south  of  the  middle  point  of  the  line  joining 
7  Orionis  (Bellatrix)  with  a  Canis  Minoris  (Procyon)  is  a  cluster 
[99]  visible  to  the  naked  eye,  and  very  pleasing  with  a  low  power. 
Some  of  the  faintest  stars  are  arranged  in  straight  lines. 

A  line  from  Sirius  to  6  Canis  Majoris,  when  prolonged  three 
fourths  of  its  length,  reaches  a  brilliant  coarse  cluster  [101],  in  a 
"  superb  "  neighborhood. 

There  is  a  fine  field  one  fifth  of  the  way  from  1 1  to  8 ;  the  fifth 
magnitude  star  10  is  in  it. 

446.  Ophiuchus,  the  Serpent  Bearer.  [  Map  IV.  ]  —  Ophiuchus  lies 
between  Hercules  and  Scorpio.  The  two  portions  of  Serpens  lie 
respectively  at  the  east  and  west  sides  of  this  constellation.  Ophiu- 
chus is  represented  as  standing  on  the  Scorpion  and  grasping  the 
Serpent  with  both  hands. 

A  line  from  Polaris  to  j3  Draconis  (in  the  Dragon's  head),  pro- 
longed an  equal  distance,  ends  near  a,  which  is  in  the  head  of  Ophi- 
uchus and  near  a  Herculis.  (3  and  7  mark  his  right  shoulder,  t  and  K 
the  left;  v  and  T  are  in  his  right  hand,  8  and  e  in  his  left.  His  right 
knee  contains  77  and  his  left  f.  The  right  foot  is  at  0,  the  left  at  p. 

The  parallelogram  (nearly)  formed  by  £  and  X  Ophiuchi  with  a 
and  p  Serpentis  is  shown  by  the  dotted  lines  on  the  map,  and  is  note- 
worthy to  the  eye :  one  diagonal  of  it  contains  five  bright  stars. 

X  [51]  is  a  binary,  having  a  period  of  about  230  years.  The  com- 
panion is  of  the  sixth  magnitude,  and  is  now  (1896)  i".7  distant. 

36  [53]>  a  fifth  magnitude  star  in  the  southernmost  part  of  the 
constellation,  11°  east  of  a  Scorpii  (Antares),  has  a  sixth  magnitude 
companion  at  a  distance  of  5". 

70  [58],  4°. 5  east  of  7,  is  a  fine  binary,  completing  a  revolution 
in  less  than  a  century :  the  seventh  magnitude  companion  is  reddistu 
The  distance  is  now  (1896)  2". 

p  [49],  in  the  left  foot,  has  an  eighth  magnitude  companion  at 
a  distance  of  4". 

A  cluster  [120]   3'  in  diameter  lies  9°. 5  due  east  of  a  Scorpii 


288  DESCRIPTIVE    ASTRONOMY. 

(Antares),  nearly  in  line  with  36.  There  are  a  number  of  other 
clusters  in  the  vicinity. 

3°  south  and  i°  west  of  f  lies  a  cluster  [i  17]  5'  in  diameter. 

One  third  of  the  way  from  e  to  /3  lies  a  cluster  [119]  8'  in  di- 
ameter, in  the  centre  of  which  the  stars  are  very  closely  crowded. 
A  line  from  <r  to  ft  prolonged  2\  times  its  former  length  strikes  a 
large  coarse  cluster  [128]. 

447.  Orion.  [Map  II.]  —  Orion  is  the  finest  constellation  in  the 
heavens,  and  strikes  the  eye  at  once :  it  is  best  seen  in  the  early 
evening  in  midwinter.  The  mighty  hunter  stands  in  the  attitude  of 
smiting  Taurus.  His  belt  is  formed  of  three  second  magnitude  stars, 
8,  e,  and  f ;  it  is  about  3°  in  length,  and  has  been  called  the  Ell  and 
Yard.  Below  it  dangles  the  sword,  composed  of  three,  or  to  good 
eyes  four,  stars  in  line.  The  shoulders  are  marked  respectively  by 
a  (Betelgueuse)  and  7  (Bellatrix).  In  the  head  is  a  small  isosceles 
right  triangle.  The  left  foot  is  marked  by  /3  (Rigel),  a  bluish  white 
star  of  the  first  magnitude  :  K  occupies  the  right  knee.  The  right  arm 
and  club,  with  which  he  is  to  smite  Taurus  full  in  the  face,  are  indi- 
cated by  the  dotted  lines  going  upward  from  a.  The  left  arm  with 
which  he  holds  up  the  skin  of  the  Nemaean  lion,  is  similarly  out- 
lined by  a  dotted  line. 

ft  [15]  has  a  ninth  magnitude  companion  at  a  distance  of  10".  It 
is  not  hard  to  see  with  a  four-inch  glass,  under  good  atmospheric 
conditions,  and  is  itself  a  very  close  double. 

f  [20]  is  a  triple,  having  a  sixth  magnitude  companion  2". 6  dis- 
tant, and  one  of  the  ninth  magnitude  57"  away. 

i  [17],  the  southernmost  star  in  the  sword,  has  an  eighth  magni- 
tude companion  at  a  distance  of  12",  and  one  of  the  tenth  magnitude 
49"  distant. 

X  [16],  in  the  head,  has  a  companion  of  the  sixth  magnitude, 
4"  distant. 

cr  [18]  is  a  triple  star,  having  a  seventh  magnitude  companion  at  a 
distance  of  42",  and  one  of  the  eighth  magnitude  12"  distant:  near 
by  is  a  small  triangle  of  three  eighth  magnitude  stars. 

i°  south  of  v,  in  the  right  hand,  is  a  cluster  [98]  of  30  stars  of  the 
ninth  magnitude  or  fainter. 

A  brilliant  field  [95]  lies  i°  north  of  0,  containing  quite  a  number 
of  stars  of  the  sixth  and  seventh  magnitudes. 


THE    CONSTELLATIONS    IN    DETAIL.  289 

In  the  sword  is  the  multiple  star  0,  surrounded  by  the  Great 
Nebula  [94],  the  finest  object  of  its  kind  in  the  sky.  See  §  390. 
In  a  four-inch  telescope  the  central  portion,  around  the  Trapezium, 
can  be  well  seen,  in  the  absence  of  the  moon. 

448.  Pegasus,  the  Winged  Horse.     [Maps  V.  and  L] — The  chief 
configuration  of  this  constellation  is  a  large  rude  square  which  is 
in  the  body  of  the  horse.     A  hook-shaped  figure  starting  from  one 
corner  of  the  square  makes  the  neck  and  head  of  the  animal.     One 
corner  of  the  square  is  found   by  drawing  a  line  from   Polaris  to 
y3  Cassiopeiae,  and  prolonging  it  an  equal  distance.     The  star  thus 
found  is  really  a  Andromedae,  but  has  at  times  been  called  S  Pegasi. 
The  neck  starts  from  the  opposite  corner  of  the  square,  and  em- 
braces the  stars  f  and  f ;  the  head  starts  at  0,  and  e  is  in  the  nose. 

K  [79],  which  is  1 6°  due  north  of  e,  has  a  companion  of  the  elev- 
enth magnitude  12"  distant.  The  main  star  is  a  very  close  double. 

A  line  from  0  to  e,  prolonged  two  thirds  of  its  length,  reaches  a 
condensed  globular  star  cluster  [137],  3'  or  4'  in  diameter. 

Midway  between  e  and  0  is  a  bright  group  [140]. 

449.  Perseus.     [Map  I.]  —  Perseus  lies  between  Auriga  and  An- 
dromeda,    a,  its  chief  star,  lies  on  a  line  from  /3  Andromedae  to  7 
Andromedae,  prolonged  i^  times  its  own  length.     The  most  striking 
configuration  is  the  trapezoid  of  which  a  is  one  vertex,  from  which 
springs  a  curved  line  of  stars  shown  by  the  dotted  line  on  the  map. 
9°  south  of  L  (which  is  in  one  corner  of  the  trapezoid)  lies  /3  (Algol), 
the  wonderful  variable  described  in  §  379.    Near  Algol  are  a  few 
stars  which  form  the  head  of  Medusa,  the  Gorgon  which  Perseus  slew. 
10°  southeast  of  Algol  lie  a  few  scattered  stars  which  complete  the 
constellation :  there  is  no  resemblance  to  the  figure  of  a  man. 

e  [12]  has  a  lilac  companion  of  the  eighth  magnitude,  at  a  dis- 
tance of  8". 

£  [10]  has  three  companions  of  the  ninth,  tenth,  and  tenth  magni- 
tudes, respectively,  at  distances  of  13",  90",  and  122". 

r\  [9]  has  an  eighth  magnitude  companion  at  a  distance  of  28''. 

Just  south  of  the  middle  point  of  a  line  from  8  Cassiopeiae  to  7 
Persei  is  a  large  hazy  spot,  visible  to  the  naked  eye  even  in  strong 
moonlight.  It  is  a  double  cluster  [85],  the  finest  object  of  its  class 
in  the  northern  hemisphere.  The  lowest  power  should  be  used  in 
viewing  it. 

19 


2QO  DESCRIPTIVE    ASTRONOMY. 

i°  north  of  a  point  five  eighths  of  the  way  from  7  Andromedae 
to  ft  Persei  (Algol)  is  one  of  the  finest  of  low-power  fields. 

450.  Pisces,  the  Fishes.  [Maps  V.,  II.,  and  I.]  —  One  of  the 
Fishes,  which  is  marked  by  a  six-sided  polygon,  is  located  just  south 
of  the  square  of  Pegasus.  The  star  i  in  this  figure  forms  nearly  an 
isosceles  right  triangle  with  the  two  stars  a  and  7,  which  form  the 
southern  side  of  the  square.  Thence  a  ribbon,  represented  on  the 
map  by  a  row  of  stars  connected  by  a  dotted  line,  extends  eastward 
to  a,  just  east  of  the  head  of  Cetus,  thence  northward  to  the  other 
Fish,  which  is  an  insignificant  and  chiefly  imaginary  creature,  the 
mouth  of  which  is  near  ft  Andromedae.  Though  none  of  the  stars  are 
especially  bright,  they  are  in  a  dull  region,  and  so  are  easily  traced. 

A  line  drawn  from  Polaris  through  ft  Cassiopeiae  to  a  Androm- 
edae (in  one  corner  of  the  square  of  Pegasus),  and  prolonged  nearly 
one  half  of  its  former  length,  terminates  close  by  the  vernal  equi- 
nox, east  of  the  hexagon  which  marks  the  southern  one  of  the  two 
Fishes. 

a  [5]  has  a  companion  of  the  fourth  magnitude,  distant  3". 

?  [2],  12°  east  and  5°  north  of  a,  has  an  eighth  magnitude  com- 
panion 23"  away. 

451.  Piscis  Australia,  the  Southern  Fish.     [Map  V.]  —  Prolong  the 
line  of  the  western  edge  of  the  square  of  Pegasus  southward,  until 
the  prolongation  is  four  times  the  length  of  the  original  line,  and  a 
(Fomalhaut)  will  be  reached  :  it  is  of  the  first  magnitude.    The  other 
stars  of  the  constellation  are  then  found  readily.     The  constellation 
is  too  far  south  for  good  telescopic  views. 

452.  Sagitta,  the  Arrow.     [Map  V.]  —  This  constellation  is  just 
north  of  Aquila  and  south  of  Vulpecula.     It  is  a  fair  representation 
of  an  arrow,  the  butt  of  which  is  marked  by  the  pretty  pair  a  and  /3, 
which  lie  midway  between  ft  Cygni  and  a  Aquilae  (Altair).     The 
point  of  the  arrow  is  at  77. 

f  [70]  has  a  companion  of  the  ninth  magnitude  9"  distant.  The 
large  star  is  a  very  close  double. 

0  [71]  has  two  companions,  one  of  the  ninth  magnitude  at  a  dis- 
tance of  1 1",  and  one  of  the  eighth,  70"  distant.  The  colors  of  the 
three  stars  are  called  pale  topaz,  gray,  and  pearly  yellow. 

About  a  degree  south  of  ft  lies  a  double  [67]  composed  of  a  ruby 
star  of  the  ninth  magnitude,  and  a  blue  star  of  the  tenth  magnitude, 
20"  distant. 


THE  CONSTELLATIONS  IN  DETAIL.  2QI 

Midway  between  7  and  S  is  a  faint  but  very  condensed  cluster  [133]. 
??  lies  in  a  beautiful  low-power  field  [135],  in  which  are  a  number 
of  doubles. 

453.  Sagittarius,  the  Archer.     [Maps  V.  and  IV.]  — The  conspicu- 
ous part  of  this  constellation  looks  like  a  bent  bow,  with  the  point  of 
the  arrow  just  west  of  its  centre,  and  the  butt  2-|-  times  as  far  east,  in 
one  corner  of  a  bright  quadrilateral.     Sagittarius  is  a  Centaur ;  the 
two  southern  stars  of  the  quadrilateral  are  in  his  body.     The  naked- 
eye  double,  0,  far  to  the  south,  not  on  the  map,  marks  one  of  his 
front  hoofs. 

A  line  from  Polaris  through  Vega,  prolonged  i^  times  its  former 
length,  strikes  the  quadrilateral.  The  winter  solstice  lies  2^°  south 
and  2°  west  of  ^,  and  is  i°  north  of  the  naked-eye  cluster  [124]. 

fji  [59],  in  the  northwest  part  of  the  constellation,  has  two  com- 
panions of  the  ninth  and  tenth  magnitudes,  at  respective  distances  of 
40"  and  45". 

Midway  between  p  and  <r  is  a  cluster  [130],  8'  in  diameter,  sur- 
rounded by  five  stars  irregularly  placed.  It  shows  well  with  a  four- 
inch  glass. 

A  line  from  a  to  X  prolonged  three  fourths  of  its  length  termi- 
nates just  south  of  a  splendid  portion  [124]  of  the  Milky  Way, 
which  well  repays  examination  by  its  richness. 

3°  north  of  p  and  i°  east  is  an  offshoot  [126]  of  the  Milky  Way, 
which  shows  a  fine  field  with  a  low  power.  2°  north  of  p  and  5° 
east  is  a  brilliant  region  [129]  visible  to  the  naked  eye. 

4°  north  and  ij°  east  of  /JL  is  a  very  rich  field  [127]. 

A  line  from  cr  to  \&  prolonged  three  eighths  of  its  length  termi- 
nates at  a  good  low-power  field  [122]  containing  about  100  stars 
from  the  ninth  magnitude  down. 

The  line  from  cr  to  X  prolonged  an  equal  distance  stops  just  south 
of  a  pair  of  fifth  magnitude  stars :  close  by  the  northern  one  is  the 
Trifid  Nebula  [123]  described  in  §  391.  A  large  telescope  is  re- 
quired to  see  it  well. 

454.  Scorpio,  the  Scorpion.     [Map  IV.] — A  line  from  Polaris  to 
0  Herculis,  prolonged  two  thirds  of  its  former  length,  strikes  a  ( An- 
tares),  a  star  of  the  first  magnitude.     The  downward  curve  from  a 
is  easily  followed  by  the  eye.     At  the  west  of  Antares  the  stars  yS, 
£,  and  TT  form  a  fine  curve,  like  the  blade  of  a  scythe,  one  of  the 
handles  of  which  is  at  a. 


292  DESCRIPTIVE    ASTRONOMY. 

a  [50]  is  an  elegant  double,  having  a  seventh  magnitude  com- 
panion less  than  4"  distant. 

ft  [46]  has  a  fifth  magnitude  companion,  13"  distant. 

v  [47],  near  ft,  has  a  seventh  magnitude  companion  40"  distant: 
each  is  a  close  double. 

f  [45]  has  a  companion  of  the  seventh  magnitude,  7"  away.  The 
large  star  is  also  double,  and  may  be  seen  elongated  with  a  four-inch 
telescope  without  difficulty. 

or  [48]  has  a  plum-colored  companion  of  the  ninth  magnitude, 
20"  distant. 

The  most  condensed  mass  of  stars  [116]  in  the  heavens  is  situ- 
ated half  way  between  a  and  ft :  it  lies  in  a  beautiful  field,  and  looks 
like  a  comet  through  a  small  telescope. 

455.  Sculptor,  the  Sculptor.     [Maps  II.  and  V.]  — This  constellation 
lies  south  of  ft  Ceti  and  east  of  a  Piscis  Australis  (Fomalhaut).     It 
is  an  insignificant  group. 

456.  Scutum,  the  Shield.     [Map  V.]  —  Scutum  is  sometimes  called 
Clypeus  Sobieskii,  the  Shield  of  Sobieski ;   it  is  small  and  inconspic- 
uous, but  lies  in  the  thick  of  the  Milky  Way :   a  line  from  Polaris  to 
a  Lyrae  (Vega),  when  prolonged  nine  tenths  of  its  former  length, 
ends  in  Scutum,   near   the    brightest  star.      There   are   many   faint 
doubles  and  rich  fields. 

457.  Serpens,  the  Serpent.     [Map  IV.]  — The  head  of  the  Serpent 
is  a  triangular  figure  just  south  of  Corona,  between  Hercules  and 
Bootes.     Thence  the  Serpent's  body  extends  southward  through  the 
conspicuous  parallelogram  described  in  §  446,  across  Ophiuchus,  east 
and  northeast,  following  the  dotted  line  on  the  map,  till  it  terminates 
at  0,  nearly  three  fourths  of  the  way  from  ft  Ophiuchi  to  8  Aquilae. 

5  [43],  near  the  head,  has  a  companion  of  the  fifth  magnitude, 
3".6  distant. 

6  [64]    has  a  companion   of  nearly   the   same    magnitude,   22" 
distant. 

Close  by  the  star  5,  which  forms  a  nearly  equilateral  triangle  with 
€  and  fJL  in  the  quadrilateral,  is  a  rich  and  condensed  cluster  [115]. 

458.  Sextans,  the  Sextant.     [Map  III.] — Sextans  is  an  insignifi- 
cant group  lying  south  of  the  Sickle.      A  line  from   rj  Leonis   to 
Regulus,  prolonged  2\  times  its  former  length,  nearly  strikes  15,  the 
brightest  star  in  the  constellation. 


THE    CONSTELLATIONS    IN    DETAIL.  293 

Half  a  degree  north  of  the  middle  point  of  a  line  joining  8  and  22, 
in  the  southwest  corner  of  the  constellation,  is  a  narrow  nebula  [107] 
5'  long,  having  a  bright  nucleus. 

459.  Taurus,   the   Bull.      [Map    II.]  —  The    face    of  the    Bull    is 
marked  by  a  V-shaped  figure   containing   the   red   first  magnitude 
star  a  (Aldebaran),  which  is  nearly  pointed  at  by  the  belt  of  Orion. 
Sirius  is  as  far  from  the  belt  on  one  side  as  Aldebaran  is  on  the 
other.     The  horns  of  the  animal  are  very  long,  their  tips  being  at 
/3  and   f.      The  well  known  cluster  of  the  Pleiades  is  in  his  fore 
shoulder.     Though  the  latter  half  of  his  body  is  missing,  he  makes  a 
brave  feint  of  charging  upon  Orion.    The  V  is  known  as  the  Hyades  : 
one  of  its  stars,  6,  is  a  naked-eye  double. 

a  [13]  has  a  tenth  magnitude  companion  at  a  distance  of  113". 
The  Crab  Nebula  [92]  lies  i°  northwest  of  £     Through  a  small 
telescope  it  is  a  simple  oval. 

460.  Triangulum,  the  Triangle.     [Map  I.]  —  The  three  bright  stars 
of  this  constellation  form  a  right  triangle,  immediately  north  of  the 
triangle  in  the  head  of  Aries. 

6,  or  i  [7],  nearly  south  of  /3,  at  a  distance  of  5°,  is  a  u  topaz- 
yellow  "  star  of  the  fifth  magnitude,  and  has  a  bluish  companion  of 
the  seventh  magnitude,  3^.5  distant. 

461.  Ursa  Major,  the  Great  Bear.     [Map  I.]  —After  the  Great  Dip- 
per has  been  learned,  the  rest  of  the  constellation  can  be  made  out 
by  the  help  of  the  dotted  lines  on  the  map.     The  stars  //,  v,  and  o 
form  the  head :  i  and  K  mark  one  of  the  fore  feet :  X  and  /*  are  in 
one  of  the  hind  feet,  v  and  ?  in  the  other.     The  stars  in  the  Great 
Dipper   have   the  following   names   from  a   to  77 :     Dubhe,    Merak, 
Phecda,   Megrez,  Alioth,   Mizar,  and    Benetnasch.     The  small  star 
near  Mizar  is  called  Alcor. 

£  [38]  (Mizar)  has  a  companion  of  the  fifth  magnitude,  14" 
distant. 

f  [31]  is  a  rather  close  and  rapid  binary,  having  a  period  of  only 
6 1  years ;  the  companion  is  of  the  fifth  magnitude. 

10°  north  of  v  and  ij°  nearly  east  of  the  fifth  magnitude  star  d  is 
a  double  nebula  [105,  106],  one  component  of  which  is  fairly  bright: 
they  are  half  a  degree  apart. 

A  line  from  a  to  7,  prolonged  three  fourths  of  its  own  length, 
strikes  a  large  oval  nebula  [no]. 


294  DESCRIPTIVE    ASTRONOMY. 

462.  Ursa  Minor,  the  Little  Bear.     [Map  I.]  —  Polaris  is  the  bright- 
est star,  and  is  in  the  end  of  the  tail.    The  stars  /3,  7,  f,  and  rj  are  in 
the   Bear's   body,  and  form  the  bowl  of  the    Little    Dipper.     The 
length  of  the  tail  may  be  ascribed  to  adaptation  to  environment. 

a  (Polaris)  [3]  has  a  companion  of  the  ninth  magnitude,  19" 
distant. 

463.  Virgo,  the  Virgin.      [Maps  IV.  and  III.]  —  The  head  of  the 
Virgin    is    5°   south    of  /3   Leonis    (Denebola).     Thence    the    body 
stretches  east  and  south  to  Libra.     The  lines  on  the  map  show  its 
general   contour.      The  right   arm  is   graciously   extended   to   take 
in  e,  and   the   left  hand   is  given  to  a  (Spica),  a  star  of  the  first 
magnitude. 

The  autumnal  equinox  lies  i°  south  of  the  middle  point  of  a  line 
connecting  /3  and  77. 

7  [35]  is  a  fine  binary  having  a  period  of  185  years:  the  com- 
ponents are  equal  in  magnitude,  and  are  now  (1896)  5"  apart. 

6°  north  and  4°  west  of  Spica  is  the  triple  star  0  [37]  ;  its  com- 
panions are  of  the  ninth  and  tenth  magnitudes,  at  distances  of  f 
and  65 "  respectively. 

In  the  wonderful  nebulous  region  of  Virgo,  bounded  by  the  stars 
/3,  77,  7,  S,  e,  and  ft  Leonis,  the  sky  is  crowded  with  nebulae,  most  of 
which  are  too  faint  for  small  telescopes.  One  of  the  brighter  ones 
[in]  is  west  of  e  and  8,  forming  with  them  an  equilateral  triangle. 

464.  Vulpecula,  the  Fox.     [Map  I.] — Vulpecula  contains  one  star 
of  the  fourth  magnitude,  which  is  3  J°  south  of  /3  Cygni  in  the  foot  of 
the  Cross.     The  rest  of  the  stars  are  fainter,  and  most  of  them  lie 
east  of  the  fourth  magnitude  star,  being  bounded  by  Delphinus  and 
Sagitta  on  the  south,  Cygnus  on  the  north,  and  Pegasus  on  the  east. 

3j°  due  north  from  7  Sagittae,  nearly  in  line  with  6  Vulpeculae 
(the  brightest  star)  and  7  Delphini,  the  Dumb-bell  Nebula  [154]  is 
located :  a  description  of  it  has  been  given  in  §  391. 


USE    OF   A    STAR    FINDER    OR    OF   AN    EQUATORIAL. 

465.  Graduation  of  the  Circles.  —  In  §§  8-12,  directions  have  been 
given  for  finding  many  objects  of  telescopic  interest  by  the  aid  of 
the  maps.  It  is  often  more  convenient  to  find  them  by  means 
of  a  star  finder  (Fig.  194),  or  of  a  telescope  equatorially  mounted 


THE  CONSTELLATIONS  IN  DETAIL. 


295 


and  provided  with  an  hour  circle  and  a  declination  circle  (Fig.  195). 
Such  circles  can  be  affixed   to  almost  any  telescope  mounting  which 


Fig.  194.  —  THE  STAR  FINDER. 

is  destitute  of  them  by  a  bright  boy  of  a  mechanical  turn  of  mind. 
Two  opposite  points  of  the  hour  circle  (the  lower  one  in  Fig.  194) 
may  be  marked  o  h.  and  12  h. 
respectively.  Each  half  of  the  cir- 
cle would  then  read  o,  I,  2,  3,  ... 
12  h.  The  circle  may  then  be  sub- 
divided into  five  minute  spaces.  It 
is  well  to  mark  two  opposite  points 
of  the  declination  circle  (the  upper 
one  in  Fig.  194)  o°,  and  to  run  the 
graduations  each  side  of  o°  up  to 
90°.  Each  whole  degree  should 
be  indicated.  The  cut  of  the  star 
finder1  shows  that  it  is  like  an 
English  equatorial  (§  44),  a  stick 
taking  the  place  of  the  telescope. 
When  the  stick  or  telescope  lies  in  the  plane  of  the  meridian, 
and  is  perpendicular  to  the  polar  axis,  the  pointer  on  each  circle 

1  A  detailed  description  of  this  instrument,  together  with  Prof.  Wm.  A.  Rogers's 
method  of  tracing  the  constellations  by  its  aid,  is  given  in  the  "  Sidereal  Messenger " 
(published  by  W.  W.  Payne,  Northfield,  Minn.)  for  April,  1889. 


Fig.  195.  —  THE  DECLINATION  CIRCLE. 


296 


DESCRIPTIVE    ASTRONOMY. 


should  be  opposite  the  zero  of  the  circle.  Both  circles  of  the  star 
finder  are  fast  to  the  polar  axis.  Any  object  in  the  lists  at  the  end 
of  this  chapter  may  be  found  by  the  star  finder,  or  an  equatorial 
telescope,  if  the  sidereal  time  is  known,  as  will  be  explained  in  the 
following  sections. 

466.  The  Sidereal  Time  at  any  Instant.  —  The  Nautical  Almanac 
gives  data  and  rules  for  finding  with  precision  the  sidereal  time  at 
any  instant,  when  the  mean  time  is  known.  The  time  may  be  ob- 
tained with  sufficient  accuracy  for  present  purposes  by  means  of  the 
following  table  and  its  accompanying  explanations. 

SIDEREAL  TIME  AT  MEAN  NOON. 


Jan. 

i, 

18 

h. 

44m. 

tt 

1  6, 

I9 

h. 

43m. 

Feb. 

i, 

20 

h. 

47m. 

tt 

1  6, 

21 

h. 

46  m. 

March  i, 

22 

h. 

37m. 

« 

16, 

23 

h. 

36111. 

April 

i, 

O 

h. 

39  m. 

tt 

16, 

I 

h. 

38  m. 

May 

i, 

2 

h. 

37m. 

it 

16, 

3 

h. 

37  m. 

June 

i. 

4 

h. 

40  m. 

tt 

1  6, 

5 

h. 

39  m. 

July 

i, 

6 

h. 

38  m. 

U 

1  6, 

7 

h. 

37m. 

Aug. 

i, 

8 

h. 

40  m. 

it 

1  6, 

9 

h. 

39  m. 

Sept. 

i, 

10 

h. 

42  m. 

tt 

16, 

ii 

h. 

42  m. 

Oct. 

i, 

12 

h. 

41  m. 

tt 

1  6, 

*3 

h. 

40  m. 

Nov. 

i, 

14 

h. 

43m. 

u 

1  6, 

15 

h. 

42  m. 

Dec. 

i, 

16 

h. 

41  m. 

tt 

1  6, 

17 

h. 

40  m. 

For  any  date  not  given  in  the  table,  subtract  the  last  preceding 
tabular  date  from  the  given  date,  multiply  the  difference  by  4  m., 
and  add  the  product  to  the  time  given  opposite  the  tabular  date 
used. 

If  the  sidereal  time  at  mean  noon  is  required  for  March  27,  the 
last  preceding  tabular  date  is  March  16;  the  difference  between  the 
dates  is  n  days:  n  X4m.  —  44m.,  which  added  to  23  h.  36m.  (the 
time  given  opposite  March  16)  gives  24  h.  20  m.  As  24  h.  is  identi- 
cal with  O  h.,  we  call  the  answer  o  h.  20  m.  This  then  is  the  reading 
of  a  sidereal  clock  at  noon  on  March  27. 

To  find  the  sidereal  time  at  gh.  23  m.  P.  M.,  we  reason  that,  if  the 
sidereal  time  at  noon  was  oh.  20 m.,  and  gh.  23m.  have  elapsed 


THE    CONSTELLATIONS    IN    DETAIL.  297 

since  then,  the  sidereal  time  will  be  found  by  adding  9  h.  23  m.  to 
o  h.  20  m.,  giving  9  h.  43  m.  for  the  time  sought.1 

To  find  the  sidereal  time  at  /h.  42m.  P.M.,  on  December  21,  we 
reason  as  follows.  At  noon  of  December  16  it  was  17  h.  40  m. ; 
December  21  is  five  days  thereafter:  5  X  4m.  =  20 m.,  which  added 
to  I7h.  40  m.  gives  i8h.  om.  as  the  sidereal  time  at  noon  of 
December  21.  Since  7h.  42m.  have  elapsed  since  noon,  we  add 
7  h.  42m.  to  i8h.  om.,  obtaining  25  h.  42  m.,  which  is  equivalent 
to  I  h.  42  m. 

To  find  the  sidereal  time  at  3  h.  10  m.  A.  M.,  October  24,  we 
first  notice  that  the  last  preceding  noon  was  October  23.  The 
sidereal  time  at  noon  of  October  23  was  I3h.  40  m.  +  7x4111., 
which  equals  14  h.  8  m.  The  interval  of  time  between  noon  of 
October  23  and  3  h.  10  m.  A.  M.  of  October  24  was  15  h.  10  m., 
which  added  to  14  h.  8  m.  gives  29  h.  18  m.  Since  this  sum 
is  over  24  h.  we  subtract  that  from  it,  and  get  5  h.  18  m.  for  the 
time  sought. 

467.  The  Hour  Angle  of  a  Star  at  any  Instant.  We  can  find  this 
if  we  know  the  right  ascension  of  the  star,  and  the  sidereal  time  at 
the  instant  at  which  the  hour  angle  is  desired.  Suppose  that  it  is 
required  to  find  the  hour  angle  of  a  Geminorum  at  8  h.  15  m.  P.  M., 
on  February  12.  From  the  table  in  §  466  we  find  that  the  sidereal 
time  at  noon  on  February  12  was  21  h.  31  m.  Then  at  8  h.  15  m. 
it  would  be  5  h.  46  m.,  as  explained  in  the  preceding  section.  The 
right  ascension  of  a  Geminorum  is  7  h-  28  m.,  as  given  in  the  list 
of  double  stars  at  the  end  of  this  chapter.  From  the  discussion  in 
§  131,  we  see  that  the  hour  angle  of  a  star  at  any  instant  is  the  dif- 
ference between  its  right  ascension  and  the  sidereal  time  at  that 
instant.  The  difference  between  5  h.  46  m.  and  7  h.  28  m.  is  I  h. 
42  m.,  which  is  the  east  hour  angle  of  the  star.  If  the  sidereal 
time  had  been  loh.  41  m.,  the  hour  angle  of  the  star  would  have 
been  3  h.  13  m.,  and  the  star  weuld  have  been  west  of  the  meridian, 
as  explained  in  §  131. 

Astronomers  use  the  following  rule  for  computing  the  hour  angle 
of  a  star  at  any  instant. 

1  This  reasoning  is  not  strictly  correct,  because  sidereal  hours  are  not  quite  of  the 
same  length  as  mean  hours.  As  a  sidereal  clock  goes  faster  than  a  mean  time  clock,  it 
will  tick  off  more  than  9  h.  while  a  mean  time  clock  is  measuring  9  h. 


298  DESCRIPTIVE    ASTRONOMY. 

Sztbtract  the  star's  right  ascension  from  the  sidereal  time  at  the 
instant.  If  the  remainder  is  positive,  the  star  is  west  of  the  merid- 
ian :  if  negative,  the  star  is  east  of  the  meridian. 

Thus,  if  the  star's  right  ascension  is  1 1  h.  41  m.,  and  the  sidereal 
time  8  h.  50  m.,  the  subtraction  gives  — 2  h.  5 1  m.,  and  the  star  has  an 
east  hour  angle  of  2  h.  51  m.  Had  the  sidereal  time  been  13  h.  5  m.', 
the  subtraction  would  have  given  -f-i  h.  21  m.,  which  would  have 
been  the  west  hour  angle  of  the  star. 

468.  Practical  Directions.  —  In  order  to  find  an  object  with  an  equa- 
torial, or  star  finder,  it  will  be  advantageous  to  give  heed  to   the 
following  detailed  directions,  which  are  based  upon  the  articles  im- 
mediately preceding :  — 

I.  Look  up  the  right  ascension  and  declination  of   the  object 
sought. 

II.  Turn  the   telescope  about  the  declination  axis  until  the  read- 
ing of  the  declination  circle  equals  the  declination  of  the  object. 

III.  Compute   the   sidereal   time :    also    the    hour  angle  of  the 
object. 

IV.  Turn  the  instrument  about  the  polar  axis,  not  disturbing  the 
reading  of  the  declination  circle,  until  the  reading  of  the  hour  circle 
corresponds  to  the  hour  angle  just  computed. 

V.  An  eyepiece  of  low  power  should  be  on  the  telescope.     If 
the  object  is  not  in  the  field  of  view,  move  the  instrument  to  and  fro 
a  little  around  the  polar  axis. 

469.  Lists  of  Telescopic  Objects.  —  The  following  telescopic  objects 
have  been  selected  because  they  can  be  seen  with   small  telescopes. 
Everything  in  the  list  will  yield  to  a  four-inch  telescope  :   a  three-inch 
will  show  most  of  them.     The  right  ascensions  and  declinations  are 
given  for  the  year  1900. 


APPENDIX    I. 


APPENDIX    I. 

470.    NAMES   OF   STARS. 

THE  following  list  contains  the  proper  names  of  some  of  the  prominent 
stars,  together  with  their  corresponding  designations  in  the  Greek  letter 
nomenclature. 


A-cher'-nar a  Eridani 

Al-br-re-o #  Cygni 

Al-cy'-o-ne T\  Tauri 

Al-deb'-a-ran a  Tauri 

Al'-ge-nib y  Pegasi 

Ar-ge-nib  (sometimes)      .     .     .      a  Persei 

AF-gol £  Persei 

Ar-i-oth e  Ursae  Majoris 

Al'-kaid t\  Ursae  Majoris 

Ar-phard o  Hydrae 

Al-phec'-ca a  Coronae  Bor. 

Al'-phe-ratz a  Andromedae 

Al'-tair a  Aquilae 

Ant-ar'-es  (ez) .    a  Scorpii 

Arc-tu'-rus a  Bootis 

Ar'-i-ded a  Cygni 

Bel'-la-trix y  Orionis 

Be-net'-nasch    ....       rj  Ursae  Majoris 
Betelgueuse  (Be'-tel-juz)   .     .     .    a  Orionis 

Ca-pel-la a  Aurigae 

Caph £  Cassiopeiae 

Cas'-tor a  Geminorum 

Cor  Car'-o-li a  Can.  Yen. 

Cor  Hy'-drae a  Hydras 

Cor  Le-o'-nis o  Leonis 

Cor  Ser-pen'-tis a  Serpentis 


De'-neb a  Cygni 

De-neb'- o-la )8  Leonis 

Dubx-he a  Ursae  Majoris 

E'-nif e  Pegasi 

Fomalhaut  (Fo'-mal-o)      a  Piscis  Australis. 

Gem'-ma a  Coronae 

Ham'-al a  Arietis 

Ko'-chab £  Ursae  Minoris 

Mar'-kab a  Pegasi 

Me'-grez 5  Ursae  Majoris 

MT-ra o  Ceti 

Mi'-rach )8  Andromedae. 

Ml'-zar ^"Ursas  Majoris. 

Phecx-da y  Ursas  Majoris. 

Po-la'-ris a  Ursae  Minoris 

PolMux £  Geminorum 

PrS'-cy-on a  Canis  Minoris 

Ras'-al-hag'-ue a  Ophiuchi 

Regx-u-lus a  Leonis 

Rigel  (Rf-ghel)    ......    ^8  Orionis 

Scheat £  Pegasi 

Sir'-i-us o  Canis  Majoris 

Spi'-ca a  Virginis 

Thu'-ban a  Draconis 

Ve'-ga a  Lyrae 


304  DESCRIPTIVE    ASTRONOMY. 


APPENDIX    II. 

471.    ASTRONOMICAL    CONSTANTS. 

d.  h.         m.  s. 

Sidereal  Year 3^5  6  9  8.97 

Tropical  Year 365  5  48  45-51 

Sidereal  Month 27  7  43  n-54 

Synodic  Month 29  12  44  2.68 

h.          m.  s. 

Sidereal  Day       ...     23       56        4.090  of  mean  solar  time. 
Mean  Solar  Day      .     .     24        3       56.556  of  sidereal  time. 

Obliquity  of  the  Ecliptic 23°     27'     8".o 

Constant  of  Precession t  •     5°"-264 

Constant  of  Aberration 2o"-492 

The  lengths  of  the  year,  the  obliquity  of  the  ecliptic,  and  the  constant  of 
precession  are  given  for  the  year  1900.  The  lengths  of  the  year  and  of  the 
month  are  given  in  mean  solar  time. 


PLANETARY    DATA. 


305 


APPENDIX     III. 
472.    PLANETARY    DATA. 


Planet. 

Mean  Distance, 
the  Earth's 
being  Unity. 

Mean  Dis- 
tance, Millions 
of  Miles. 

Sidereal 
Period. 

Eccentricity 
of  Orbit. 

Inclination  of 
the  Orbit  to 
the  Ecliptic. 

Mercury 

0.387099 

36.0 

87.969 

0.2056    . 

o      / 

7     o 

Venus 

0.723332 

67.2 

224.701 

0.0068 

3  24 

The  Earth 

I  .OOOOOO 

92.9 

365-256 

0.0168 

o    o 

Mars 

1.523691 

I4I-5 

686.980 

0.0933 

1  51 

Jupiter 

5.2O28OO 

483.3 

yrs. 
11.86 

0.0483 

i  19 

Saturn 

9.53886! 

886.1 

29.46 

0.0561 

2    30 

Uranus 

19.18329 

1782.1 

84.02 

0.0463 

o  46 

Neptune 

30.05508 

2792.0 

164.78 

0.0090 

i  47 

Planet. 

Mean 
Diameter 
in  Miles. 

Mass,  the 

Sun's  being 
Unity.      - 

Density, 
the  Earth's 
being 
Unity. 

Time  of 
Rotation. 

Inclination 
of  Equator 
to  Orbit. 

Superficial 
Gravity,  the 
Earth's  be- 
ing Unity. 

Mercury 
Venus 
The  Earth 
Mars 
Jupiter 
Saturn 
Uranus 
Neptune 

3.03° 
7,700 
7,918 
4,230 
88,000 

73,000 

31,900 
34,800 

1 
2,668,700 

1 

2.21 

0.86 

I.OO 

0.72 
0.24 
0.13 

O.22 
0.20 

88  days 

225  days 
h.    m.      s. 
23  56     4.09 

24  37  22.67 
9  55 

TO    14   24 

Unknown 
Unknown 

0°  (?) 
0°  (?) 

23°    27' 

24°  50' 
3°     5' 
26°  49' 
Unknown 
Unknown 

0.85 
0.83 
I.OO 
0.38 
2.65 

1.18 
0.91 

0.88 

425,000 

1 

331,100 
1 

3,104,700 

1 

1048 

1 
3486 

1 

22765 
1 

19149 

306 


DESCRIPTIVE    ASTRONOMY. 


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h-  1000   ^O  N  vo  ro  *^         f^  *O   r^vO         ^ 

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Satellite. 

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LANDMARKS    IN    THE    HISTORY    OF    ASTRONOMY.          307 


APPENDIX    IV. 

473.    LANDMARKS    IN    THE    HISTORY    OF    ASTRONOMY. 

THE  data  to  be  given  under  this  heading  may  serve  to  outline,  though  in  a 
rude  and  imperfect  way,  the  historical  development  of  the  science  of  astron- 
omy. Many  suggestions  for  essays  to  be  written  by  the  students  may  be 
derived  from  it. 

Herodotus  declared  (with  his  customary  accuracy  !)  that  the  Egyptians 
had  made  astronomical  observations  for  more  than  11,000  years,  and  had  seen 
the  ecliptic  perpendicular  to  the  equator.  Though  these  statements  are  man- 
ifestly untrue,  they  indicate  that  the  Egyptians  cultivated  astronomy  from  a 
remote  antiquity.  Diodorus  states  that  they  were  able  to  calculate  eclipses. 
Their  year  consisted  of  365  days  :  religious  ceremonies  were  regulated  by 
the  phases  of  the  moon.  As  their  writings  are  lost,  the  real  extent  of  their 
knowledge  is  largely  a  matter  of  conjecture.  The  gigantic  Pyramid  of  Che- 
ops, set  square  with  the  points  of  the  compass,  silently  testifies  to  astronomical 
knowledge  on  the  part  of  its  unknown  builders. 

The  Chaldeans  lived  in  and  about  Babylon.  Porphyry  states  that  Callis- 
thenes  transmitted  to  Aristotle  a  series  of  Babylonian  observations  reaching 
back  to  2200  B.  c.  But  Ptolemy,  who  made  use  of  the  Chaldean  observations, 
quotes  none  prior  to  720  B.  c.  They  learned  to  predict  eclipses  by  means  of 
their  discovery  of  the  eighteen-year  cycle,  called  the  Saros  :  similar  series  of 
eclipses  recur  in  successive  cycles. 

The  Hindoos  seem  to  have  possessed  an  extensive  knowledge  of  astronomy 
in  olden  times.  But  the  dates  of  their  ancient  writings  are  very  uncertain. 
Some  believe  that  their  knowledge  was  derived  from  the  Greeks,  while  others 
assign  to  it  a  much  higher  antiquity.  There  are  references  in  their  writings 
to  a  conjunction  of  the  planets  which  took  place  5,000  years  ago.  It  is  sup- 
posed that  their  knowledge  of  it  was  not  obtained  by  observation,  but  by  cal- 
culating backwards.  Modern  tables  show  that  the  conjunction  was  far  from 
exact. 

The  Chinese  refer  the  beginning  of  their  astronomical  observations  to  a 
date  about  3000  B.  c.  Over  5,700  years  ago  the  Emperor  Fou-Hi  is  reputed 
to  have  been  a  diligent  student  of  astronomy.  About  2600  B.  c.  Hoang-Ti, 
likewise  an  emperor,  is  said  to  have  built  an  observatory,  and  to  have  estab- 
lished a  mathematical  tribunal  for  the  purposes  of  correcting  the  calendar  and 


308  DESCRIPTIVE    ASTRONOMY. 

predicting  eclipses.  It  is  stated  that  a  solar  eclipse,  which  occurred  some  4,000 
years  ago,  was  of  more  than  usual  interest  to  Hi  and  Ho,  two  imperial  astron- 
omers, who  failed  to  predict  it,  and  so  lost  their  heads  through  heedlessness. 
It  seems  probable  that  the  records  after  720  B.  c.  are  authentic;  They  em- 
brace accounts  of  eclipses  and  of  remarkable  comets. 

B.  C.  640-546.  Thales,  the  chief  of  the  Seven  Sages,  flourished.  He  was 
the  founder  of  the  Greek  school  of  astronomy ;  he  taught  that  the  moon 
receives  its  light  from  the  sun,  while  the  stars  are  self-luminous ;  he  believed 
that  the  earth  was  a  sphere. 

B.  C.  611-547.  Anaximander,  of  Miletus,  was  the  immediate  successor 
of  Thales  in  the  school  of  Ionian  philosophers.  He  was  distinguished  for  his 
wide  knowledge  of  astronomy  and  geography,  and  is  thought  to  have  intro- 
duced the  sun  dial  into  Greece. 

B.  C.  569-470.  Pythagoras,  of  Samos,  is  reputed  to  have  taught  his  dis- 
ciples that  the  earth  was  not  the  centre  of  the  universe,  but  that  there  was  a 
central  fire  about  which  the  sun,  moon,  earth,  planets,  and  stars  revolved. 

B.  C.  520-460.  Parmenides,  who  lived  about  this  time,  wrote  a  poem  on 
Nature,  fragments  of  which  have  come  down  to  us.  He  is  said  to  have 
taught  that  the  earth  was  a  sphere,  and  that  Lucifer,  the  morning  star,  was 
the  same  body  as  Hesperus,  the  evening  star. 

B.  C.  500-428.  Anaxagoras,  of  Clazomenae,  an  intimate  friend  of  Peri- 
cles, was  another  philosopher  of  the  Ionian  school,  who  explained  eclipses 
correctly. 

B.  C.  469-399.  Socrates  used  his  influence  against  the  study  of  astron- 
omy, except  for  the  practical  purposes  of  surveying  and  determination  of 
time.  This  was,  however,  a  mere  incident  in  his  teaching,  which  was  chiefly 
directed  to  moral  ends. 

B.  C.  460.  Diogenes,  of  Apollonia,  asserted  that  the  oblique  position  of 
the  earth's  axis  with  reference  to  the  plane  of  its  orbit  was  a  cause  of  the 
changes  of  the  seasons. 

B.  C.  433.  Meton,  an  Athenian,  discovered  the  "  Metonic  Cycle,"  which 
is  still  used  in  finding  the  time  of  Easter.  The  cycle  embraces  235  synodic 
months,  which  are  almost  exactly  equal  to  19  years  of  365!  days  each.  This 
cycle  is  also  of  use  in  predicting  eclipses. 

B.  C.  366.  Eudoxus  is  said  by  Pliny  to  have  introduced  into  Greece  the 
common  year  of  365^  days. 

B.  C.  340.  Autolycus,  of  ^Eolis,  wrote  two  astronomical  works,  the  oldest 
extant  specimens  of  astronomical  writing. 

B.  C.  330.  Pytheas,  a  noted  Greek  navigator,  pointed  out  the  fact  that 
there  was  a  connection  between  the  tides  and  the  moon. 

B.  C.  287-212.     Archimedes   made   great   strides   in   pure  and   applied 


LANDMARKS    IN    THE    HISTORY    OF    ASTRONOMY.  309 

mathematics  :  he  attempted  to  measure  the  sun's  diameter.  A  planetarium 
which  he  constructed  was  celebrated  in  its  day. 

B.  C.  280.  Aristarchus,  of  Samos,  flourished.  He  was  the  first  to  main- 
tain that  the  earth  revolved  about  the  sun  :  he  also  devised  a  correct  method 
of  determining  the  relative  distances  of  the  sun  and  moon  from  the  earth. 

B.  C.  276-196.  Eratosthenes,  of  Gyrene,  determined  the  obliquity  of  the 
ecliptic,  and  was  the  first  to  attempt  to  measure  the  magnitude  of  the  earth 
by  a  correct  method. 

^  B.  C.  190-120.  Hipparchus,  of  Nicsea  in  Bithynia,  did  the  memorable 
work  which  has  given  him  the  appellation  of  the  Father  of  Astronomy.  He 
was  the  first  to  use  right  ascensions  and  declinations,  and  made  the  earliest 
catalogue  of  stars.  The  solution  of  plane  and  spherical  triangles  by  principles 
closely  akin  to  those  now  employed  in  trigonometry,  was  first  accomplished 
by  him.  He  devised  the  method  of  locating  places  on  the  earth  by  latitude 
and  longitude,  discovered  the  precession  of  the  equinoxes,  calculated  eclipses, 
divided  the  day  into  periods  of  twelve  hours  each,  and  determined  the  pe- 
riods of  the  planets. 

A.  D.  100-170.  Ptolemy,  the  Alexandrian  astronomer,  produced  a  num- 
ber of  astronomical  and  geographical  works,  the  most  celebrated  of  which  is 
now  known  as  the  Almagest,  a  name  given  by  the  Arabians.  The  theory  of 
the  celestial  motions  which  he  advocated  is  known  as  the  Ptolemaic  theory, 
and  enthralled  astronomers  for  1,400  years.  It  placed  the  earth,  a  motion- 
less sphere,  in  the  centre  of  the  universe,  which  revolved  about  it. 

A.  D.  415.  The  modest  and  beautiful  Hypatia,  daughter  of  Theon,  an 
Alexandrian  philosopher,  was  murdered.  She  was  the  first  woman  known  to 
have  been  profoundly  versed  in  mathematics. 

A.  D.  877-929.  Albategnius,  an  Arabian  astronomer,  made  his  observa- 
tions :  he  was  the  most  accomplished  astronomer  from  the  days  of  Hippar- 
chus to  those  of  Tycho  Brahe.  He  made  a  star  catalogue,  and  obtained 
more  exact  values  of  the  annual  precession  and  of  the  obliquity  of  the  ecliptic 
than  had  been  known  previously. 

A.  D.  1214-1294.  Roger  Bacon  laid  the  foundations  of  the  modern  exper- 
imental method  in  science,  afterwards  elaborated  by  Francis  Bacon :  he  was 
a  conspicuous  champion  of  intellectual  liberty.  His  researches  in  optics,  if 
pursued  a  little  further,  might  have  led  him  to  the  invention  of  the  telescope. 

A.  D.  1288.  The  first  important  public  clock  in  England  was  erected : 
the  pendulum  was  not  yet  applied  to  timepieces. 

A.  D.  1436-1476.  John  Miiller,  of  Konigsberg,  better  known  as  Regio- 
montanus,  brought  trigonometry  to  a  high  degree  of  advancement,  wrote 
several  valuable  works,  calculated  the  places  of  the  planets  for  many  years  to 
come,  and  improved  the  imperfect  clocks  then  in  use. 


3IO  DESCRIPTIVE    ASTRONOMY. 

A.  D.  1543.  The  great  work  of  Copernicus,  entitled  De  Revolutionibus 
Orbium  Celestium,  was  published.  This  work  set  forth  the  theory  that  the 
sun  was  the  centre  of  the  solar  system.  The  theory  gained  acceptance  only 
after  a  sturdy  battle  with  the  adherents  of  the  Ptolemaic  system,  which  had 
been  generally  believed  for  fourteen  centuries. 

A.  D.  1576.  Tycho  Brahe,  a  Dane,  begins  the  construction  of  the  splen- 
did observatory  called  Uraniburg,  erected  upon  an  island  in  the  Baltic  Sea 
through  the  munificence  of  Frederick,  the  king  of  Denmark.  The  instru- 
ments which  he  employed  were  much  more  accurate  than  any  others  that  had 
ever  been  constructed.  The  study  of  his  observations  of  the  planets  led 
Kepler  to  the  discovery  of  his  famous  laws. 

A.  D.  1583.  Galileo,  of  Pisa,  noticed  that  the  vibrations  of  a  pendulum 
were  isochronous  (performed  in  equal  times).  This  he  discovered  by  ob- 
serving the  oscillations  of  a  great  bronze  lamp  suspended  from  the  ceiling  of 
the  cathedral  in  his  native  town. 

A.  D.  1596.     Fabricius  discovered  the  variability  of  Mira. 

A.  D.  1600.  Giordano  Bruno,  who  had  unsparingly  exposed  many  of  the 
absurdities  of  the  Aristotelian  system  of  natural  philosophy,  had  pointedly 
ridiculed  it,  and  had  propagated  heretical  notions,  (as,  for  instance,  that  the 
stars  were  suns,)  was  burned  at  Rome. 

A.  D.  1608.  Hans  Lippershey,  of  Middleburg  in  Holland,  invented  the 
refracting  telescope. 

A.  D.  1609.  Kepler  published  his  "  Treatise  on  the  Motion  of  the  Planet 
Mars,"  in  which  his  first  and  second  laws  of  planetary  motion  were  enun- 
ciated. Galileo  made  a  telescope  having  a  concave  lens  for  the  eyepiece : 
opera-glasses  have  such  eyepieces. 

A.  D.  1610.  Galileo  announced  that  his  telescope  had  revealed  moons 
accompanying  Jupiter,  mountains  on  the  moon,  the  phases  of  Venus,  etc. 

A.  D.  1613.  Galileo  published  his  discovery  of  spots  on  the  sun  :  by 
observations  of  them  he  detected  its  rotation. 

A.  D.  1614.  Napier,  a  Scotchman,  published  his  Mirifici  Logarith- 
morum  Canonis  Descriptio.  Though  he  is  the  inventor  of  logarithms,  those 
commonly  employed  in  calculation  are  due  to  his  friend  Briggs. 

A.  D.  1618.     Kepler  discovered  his  third  law  of  planetary  motion. 

A.  D.  1631.  The  first  recorded  transit  of  Mercury  was  observed  by  Gas- 
sendi,  one  of  the  most  eminent  of  Galileo's  disciples. 

A.  D.  1638.  Gascoigne  invented  and  used  the  filar  micrometer.  The 
present  precision  of  observations  of  the  places  of  the  celestial  bodies  is  largely 
due  to  the  use  of  this  instrument. 

A.  D.  1639.  A  transit  of  Venus  was  observed  for  the  first  time  :  Horrocks 
and  Crabtree  were  the  observers. 


LANDMARKS    IN    THE    HISTORY    OF    ASTRONOMY.  311 

A.  D.  1655.  Huyghens  discovered  that  the  mysterious  appendage  of 
Saturn  was  a  ring. 

A.  D.  1656.  Huyghens  made  the  first  pendulum  clock,  thus  giving  to 
astronomers  the  priceless  boon  of  an  accurate  instrument  for  measuring 
time. 

A.  D.  1663.  James  Gregory,  a  Scotch  professor,  invented  the  form  of  the 
reflecting  telescope  which  bears  his  name  (the  Gregorian  form) . 

A.  D.  1675.  Romer,  a  Dane,  the  inventor  of  the  transit  instrument,  an- 
nounced that  light  occupied  time  in  traversing  the  celestial  spaces,  and  deter- 
mined its  velocity  roughly. 

A.  D.  1687.  Newton  published  the  Principia,  universally  conceded  to  be 
the  masterpiece  of  the  world's  scientific  thought. 

A.  D.  1705.  Halley  predicted  that  the  Great  Comet  of  1682  would  re- 
turn in  1759. 

A.  D.  1727.  Bradley,  an  English  astronomer,  discovered  the  aberration 
of  light. 

A.  D.  1731.  Halley  invented  the  sextant?  which  has  proved  invaluable  to 
mariners. 

A.  D.  1758.  Dollond,  an  English  optician,  invented  the  form  of  achro- 
matic object-glass  now  generally  used. 

A.  D.  1765.  Harrison,  an  English  watchmaker,  finally  obtained  a  portion 
of  the  reward  offered  by  Parliament  for  improvement  in  watches  for  the 
benefit  of  navigation.  His  chronometer  ran  very  well,  but  was  much  larger 
than  the  chronometers  of  to-day.  He  was  the  inventor  of  the  "  gridiron  " 
pendulum. 

A.  D.  1781.     Sir  William  Herschel  discovered  the  planet  Uranus. 

A.  D.  1795.  Rehabilitation  of  the  French  Academy  of  Sciences,  as  a 
branch  of  the  Institute.  The  latter  part  of  the  eighteenth  century  and  the 
early  years  of  the  nineteenth  were  distinguished  by  many  profound  and  ele- 
gant mathematical  researches  concerning  the  movements  of  the  bodies  com- 
posing the  solar  system  ;  special  attention  was  paid  to  the  perturbations  due 
to  their  mutual  attractions.  Foremost  among  the  investigators  were  Laplace 
and  Lagrange,  the  most  eminent  of  French  mathematicians. 

A.  D.  1801.     Piazzi,  of  Palermo,  discovered  Ceres,  the  first  minor  planet. 

A.  D.  1803.  Sir  William  Herschel  published  his  discovery  that  certain 
double  stars  have  a  motion  of  revolution  about  their  common  centre  of 
gravity. 

A.  D.  1840.  The  moon  was  first  photographed  by  Dr.  J.  W.  Draper,  of 
New  York. 

A.  D.  1846.  The  planet  Neptune  was  discovered  :  this  is  esteemed  the 
greatest  triumph  of  mathematical  analysis. 


312  DESCRIPTIVE    ASTRONOMY. 

A.  D.  1859.  Spectrum  analysis,  which  has  lately  yielded  marvellous  re- 
sults, entered  the  service  of  astronomy. 

A.  D.  1867.  The  orbit  of  the  November  meteor  showers  was  proved  to 
be  practically  identical  with  that  of  Tempel's  comet. 

A.  D.  1868.  The  sun's  prominences  were  observed  by  Janssen  and 
Lockyer  by  means  of  the  spectroscope,  in  full  sunshine  :  they  had  hitherto 
been  seen  only  during  total  solar  eclipses. 

A.  D.  1877.  The  satellites  of  Mars  were  discovered  by  Professor  Asaph 
Hall,  with  the  twenty-six  inch  telescope  of  the  United  States  Naval  Observa- 
tory, at  Washington. 

A.  D.  1887.  An  International  Photographic  Congress,  meeting  at  Paris, 
decided  upon  a  plan  for  photographing  the  entire  heavens. 

A.  D.  1892.     The  fifth  satellite  of  Jupiter  was  discovered  by  Barnard. 

A.  D.  1895.  Saturn's  rings  were  spectroscopically  proved  by  Keeler  to 
be  composed  of  small  bodies.  Helium  was  found  to  be  widely  disseminated 
throughout  the  universe. 


TOPICS    FOR    ESSAYS.  313 


APPENDIX   V. 

474,    TOPICS    FOR    ESSAYS. 

THE  following  subjects  are  suggested  for  essays.     The  topics  cover  a  wide 
range,  and  are  of  various  degrees  of  difficulty. 

1.  The  Astronomy  of  the  Chinese. 

2.  The  Astronomy  of  the  Chaldeans. 

3.  Astronomy  among  the  Ancient  Hindoos. 

4.  The  Ancient  Greek  Astronomy  (especially  the  work  of  Thales,  Pythagoras, 
and  Hipparchus). 

5.  The  Astronomical  Work  of  Ptolemy  (explaining  particularly  the  system  of 
cycles  and  epicycles  embraced  in  the  Ptolemaic  theory). 

6.  The  Debt  of  Astronomy  to  the  Arabians. 

7.  The  Origin  of  the  Constellations. 

8.  Ancient  Ideas  of  the  Nature  and  Movements  of  the  Heavenly  Bodies. 

9.  Ancient  Ideas  of  the  Shape,  Support,  and  Motion  of  the  Earth. 
10.    The  Reckoning  of  Time  among  Ancient  Nations. 

u.  Astrology. 

12.  Astronomical  References  in  the  Bible. 

13.  Copernicus. 

14.  Tycho  Brahe. 

15.  Kepler. 

1 6.  Galileo. 

17.  Newton. 

1 8.  Laplace. 

19.  The  Herschels  (William,  Caroline,  and  John). 

20.  Growth  of  Knowledge  of  the  Planetary  Motions  (especially  the  relation 
between  the  advances  made  by  Copernicus,  Tycho,  Kepler,  and  Newton). 

21.  Invention  and  Development  of  the  Refracting  Telescope. 

22.  Invention  and  Development  of  the  Reflecting  Telescope. 

23.  Astronomical  Spectroscopy. 

24.  Astronomical  Photography. 

25.  The  Nebular  Hypothesis. 

26.  Habitability  of  other  Worlds. 

27.  Does  Astronomical  Research  tend  to  produce  Scepticism  ? 

28.  The  Characteristics  of  an  Ideal  Astronomer. 

29.  Geodesy,  or  the  Measurement  of  the  Earth's  Form  and  Dimensions. 


314  DESCRIPTIVE    ASTRONOMY. 

30.  The  Work  of  the  United  States  Coast  Survey. 

31.  The  System  of  Standard  Time  in  the  United  States,  and  its  Advantages. 

32.  Methods  of  Measuring  the  Velocity  of  Light. 

33.  The  Decay  of  the  Universe. 

34.  The  Magnitude  of  the  Forces  at  work  in  the  Sun. 

35.  Pending  Problems  in  Astronomy. 

36.  The  Making  of  a  Modern  Object-glass. 

37.  The  Stability  of  the  Solar  System. 

38.  The  Great  Telescopes  of  the  World. 

39.  The  Evolution  of  the  Moon  from  the  Earth. 

40.  Electricity  as  a  Handmaid  of  Astronomy. 

41.  Personal  Equation. 

42.  An  Ideal  Site  for  an  Observatory. 

43.  Usefulness  of  Astronomy. 

44.  The  Mental  Training  to  be  derived  from  the  Study  of  Astronomy. 

45.  Foucault's  Pendulum  Experiment. 

46.  History  of  Clocks  and  Watches. 

47.  Progress  of  Astronomy  during  the  Nineteenth  Century. 

48.  The  Moon  and  the  Weather. 

49.  The  Tides. 

50.  The  Sun  as  a  Source  of  Terrestrial  Energy. 


QUERIES    FOR    USE    IN    REVIEWS    AND    EXAMINATIONS.       315 


APPENDIX   VI. 

475.     QUERIES    FOR    USE    IN    REVIEWS    AND    EXAMINATIONS. 

THE  following  questions  are   intended  to  embrace  the  most  important 
topics  treated  in  Chapters  I.  to  XIV, 

1.  Name  the  principal  classes  of  celestial  objects. 

2.  What  is  the  celestial  sphere,  as  defined  by  mathematical  astronomers? 

3.  Define  the  celestial  poles,  the  celestial  equator,  the  zenith,  the  nadir,  and 
the  plane  of  the  horizon. 

4.  Explain  the  chief  difference  between  reflecting  and  refracting  telescopes. 

5.  Explain  the  function  of  the  object-glass,  and  of  the  eyepiece  of  a  tele- 
scope. 

6.  Give  the  meanings  of  the  terms  reflection,  refraction,  and  dispersion  of 
light. 

7.  Explain  how  a  telescope  is  made  to  follow  any  star  by  means  of  an  equa- 
torial mounting. 

8.  Give  some  hints  as  to  the  tnethod  of  using  a  small  telescope. 
tjr'State  the  distance,  diameter,  and  rotation  time  of  the  sun. 

10.  Describe  the  appearance  of  a  sun  spot,  and  tell  of  the  periodicity  and 
cause  of  these  strange  objects. 

11.  Describe  the  photosphere,  chromosphere,  prominences,  and  corona. 

12.  Describe  the  construction  of  a  spectroscope. 

13.  Give  the  laws  of  spectrum  analysis. 

14.  Give  some  illustrations  of  the  distance,  light,  and  heat  of  the  sun. 

15.  Explain  the  "contraction  theory"  of  the  maintenance  of  the  sun's  heat. 

1 6.  Explain  how  the  earth's  diameter  is  found. 

17.  Why  does  not  the  plumb-line  always  point  toward  the  earth's  centre  ? 

1 8.  Tell  how  to  draw  an  ellipse  :  define  foci,  major  axis,  minor  axis,  perihelion, 
and  aphelion. 

19.  Define  ecliptic,  equinoxes,  and  solstices. 

20.  Explain  why  the  days  are  long  in  summer,  and  short  in  winter,  in  middle 
latitudes. 

21.  Why  is  the  sun  continuously  above  the  horizon,  at  the  north  pole,  for 
six  successive  months  ? 

22.  Explain  the  two  principal  causes  of  the  changes  of  the  seasons. 

23.  Give  the  cause  of  the  precession  of  the  equinoxes,  and  draw  a  diagram 
showing  the  movement  of  the  north  celestial  pole  due  to  precession. 


316  DESCRIPTIVE    ASTRONOMY. 

24.  Show  how  the  aberration  of  light  affects  the  direction  in  which  a  tele- 
scope points. 

25.  State  the  cause  of  refraction,  and  its  effect  on  the  apparent  place  of  a 
star. 

26.  Define  the  sidereal  year  and  the  tropical  year,  and  show  why  one  is  longer 
than  the  other. 

27.  State  the  principle  according  to  which  leap  years  are  determined  in  the 
Gregorian  calendar. 

28.  Give  accurate  definitions  of  the  plane  of  the  meridian  of  any  place,  and 
of  the  latitude  and  longitude  of  the  place. 

29.  Define  celestial  meridian  (of  any  point  on  the  earth),  prime  vertical,  alti- 
tude, and  azimuth. 

30.  Define  hour  circle,  right  ascension,  declination,  north  polar  distance,  and 
hour  angle. 

31.  What  is  meant  by  the  horizontal  parallax  of  a  celestial  object  ? 

32.  What  is  the  difference  between  a  mean  solar  day  and  an  apparent  solar 
day? 

33.  State  the  causes  of  the  unequal  lengths  of  apparent  solar  days. 

34.  What  is  the  distinction  between  a  civil  day  and  an  astronomical  day  ? 

35.  Explain  the  relation  between  sidereal  time  and  right  ascension. 

36.  Describe  a  meridian  circle. 

37.  Tell  how  to  find  the  error  of  a  clock  by  observing  stars  with  a  meridian 
circle. 

38.  Prove  that  the  altitude  of  the  pole  equals  the  latitude  of  the  place  of 
observation. 

39.  Give  the  demonstration  for  finding  the  latitude  of  a  place  by  observing 
altitudes  of  the  pole  star. 

40.  How  is  the  longitude  between  two  cities  found  by  aid  of  the  telegraph  ? 

41.  How  is  the  position  of  a  ship  found  at  sea  ? 

42.  What  is  meant  by  the  sidereal  and  synodic  periods  of  the  moon  ? 

43.  State  the  causes  of  the  libration  of  the  moon. 

44.  Describe  and  explain  the  phases  of  the  moon. 

45.  Describe  the  general  characteristics  of  the  lunar  surface. 

46.  State  some  reasons  for  believing  that  the  lunar  atmosphere  is  extremely 
rare. 

47.  Give  an  explanation  of  the  disappearance  of  the  air  and  water  which  the 
moon  may  have  possessed  in  the  past. 

48.  Explain  the  cause  of  the  coldness  of  the  lunar  surface. 

49.  State  a  few  superstitions  about  the  moon,  and  tell  of  its  real  worth  to  man. 

50.  Draw  a  diagram  and  make  plain  the  meanings  of  the  umbra  and  penumbra 
of  the  shadow  of  the  earth  or  of  the  moon. 

51.  Describe  the  successive  appearances  of  the  moon  during  a  total  lunar 
eclipse. 

52.  Give  the  reasons  why  solar  eclipses  are  sometimes  partial,  sometimes  total, 
and  sometimes  annular. 


QUERIES    FOR    USE    IN    REVIEWS    AND    EXAMINATIONS.       317 

53.  Describe  the  phenomena  of  a  total  solar  eclipse. 

54.  What  observations  are  made  by  astronomers  at  the  time  of  a  total  solar 
eclipse  ? 

55.  State  Newton's  law  of  gravitation. 

56.  State  Kepler's  laws  of  planetary  motion. 

57.  Define  superior  and  inferior  planets. 

58.  Define  the  conjunction,  opposition,  elongation,  and  quadrature  of  a  planet. 

59.  Explain  why  a  superior  planet  retrogrades. 

60.  State  the  diameters,  distances    (from  the  sun),  times  of  revolution,  and 
times  of  rotation,  of  Mercury,  Venus,  the  Earth,  and  Mars. 

61.  Tell  of  the  telescopic  appearance  and  physical  condition  of  Mercury. 

62.  Tell  of  the  telescopic  appearance  and  physical  condition  of  Venus. 

63.  Describe  a  transit  of  Venus,  and  tell  the  special  use  that  astronomers 
have  made  of  such  transits. 

64.  Tell  about  the  polar  caps,  seas,  continents,  clouds,  and  atmosphere  of 
Mars. 

65.  Describe  the  canals  and  satellites  of  Mars. 

66.  State  and  comment  on  Bode's  law. 

67.  Recount  the  circumstances  of  the  discovery  of  the  first  minor  planet,  and 
the  computation  of  its  orbit. 

68.  Describe  the  present  methods  of  discovering  and  keeping  track  of  the 
asteroids. 

69.  Give  theories  of  the  origin  of  the  asteroids. 

70.  State  the  diameters,  distances  (from   the  sun),  times  of  revolution,  and 
times  of  rotation,  of  Jupiter,  Saturn,  Uranus,  and  Neptune. 

71.  Describe  the  telescopic  appearance  of  Jupiter. 

72.  Tell  of  the  atmosphere,  light,  heat,  and  physical  condition  of  Jupiter. 

73.  Describe  the  satellites  of  Jupiter  :  also  explain  their  eclipses,  occultations, 
and  transits. 

74.  Show  how  the  velocity  of  light  was  discovered  by  observations  of  Jupiter's 
moons. 

75.  Describe  the  telescopic  appearance  of  Saturn. 

76.  Narrate  the  history  of  the  discovery  of  Saturn's  rings,  and  describe  their 
changes  of  appearance. 

77.  Discuss  the  structure  and  stability  of  Saturn's  ring  system. 

78.  Tell  about  Saturn's  satellites,  and  the  physical  condition  of  the  planet. 

79.  Narrate  the  history  of  the  discovery'of  Uranus. 

80.  Tell  of  the  telescopic  appearance,    the  satellites,  and  the  physical  condi- 
tion of  Uranus. 

8 1.  Tell  the  history  of  the  discovery  of  Neptune. 

82.  Tell  of  the  telescopic  appearance,  the  satellite,  and  the  physical  condition 
of  Neptune. 

83.  Describe  the  present  methods  of  searching  for  comets. 

84.  Tell  how  comets  are  designated. 

85.  Name  and  describe  the  parts  of  a  comet. 


318  DESCRIPTIVE    ASTRONOMY. 

86.  Name  the  forms  of  the  orbits  of  comets  and  state  the  sign^ance  of 
these  forms. 

87.  Tell  about  groups  and  planet's  families  of  comets. 

88.  Describe  the  changes  in  the  appearance  of  a  comet  as  it  approaches 
the  sun. 

89.  State  the  supposed  constitution  of  the  head  and  nucleus  of  a  comet. 

90.  Describe  the  evolution  of  a  comet's  tail,  and  the  three  types  of  tails. 

91.  What  causes  give  to  comets  their  brightness  ? 

92.  Why  have  comets  been  dreaded,  and  what  occasion  is  there  for  dread  ? 

93.  Narrate  the  histories  of  three  remarkable  comets. 

94.  Describe  the  two  classes  of  meteors. 

95.  Give  an  account  of  some  noted  meteorite. 

96.  Explain   carefully  the  effect  of  the  swift  rush  of  a  meteorite  through 
the  air." 

97.  What  are  meteorites  composed  of  ? 

98.  State  the  theories  of  the  origin  of  meteorites. 

99.  Tell  how  to  observe  the  path  of  a  meteorite.. 

100.  Explain  why  more  shooting  stars  are  seen  in  the  early  morning  than  in. 
the  evening. 

101.  Tell  about  the  velocities  and  masses  of  shooting  stars. 

102.  Define  and  explain  the  radiant  of  a  meteoric  shower. 

103.  Describe  a  great  meteoric  shower. 

104.  Give  the  supposed  history  of  the  Leonids. 

105.  Tell  the  interesting  facts  about  the  Bielids. 

1 06.  State  the  relation  between  comets  and  meteors. 

107.  Describe  the  zodiacal  light,  and  give  a  theory  of  its  cause. 

1 08.  Tell  about  the  number  of  fixed  stars  visible  with  different  means,  and 
explain  their  scintillation. 

109.  Describe  the  appearance  of  the  Milky  Way. 

i  TO.  Give  the  history  of  the  naming  of  the  constellations  now  recognized. 

in.  State  the  methods  of  naming  individual  stars. 

112.  How  is  the  brightness  of  a  star  denoted? 

1 13.  How  are  the  stars  distributed  in  the  heavens? 

114.  Tell  about  star  clusters. 

115.  What  are  the  stars,  and  how  large  are  they  ? 

1 16.  Define  the  parallax  of  a  fixed  star. 

117.  Explain  how  the  distances  of  the  fixed  stars  are  found. 

1 1 8.  State  the  supposed  causes  of  the  various  colors  of  stars. 

119.  Describe  the  types  of  stellar  spectra. 

120.  Give  the  theories  as  to  the  form  of  the  visible  stellar  universe. 

121.  What  is  meant  by  the  "  proper  motions  "  of  the  stars  ? 

122.  How  are  the  velocities  of  stars  in  the  line  of  sight  found  ? 

123.  How  has  the  direction  of  the  sun's  motion  in  space  been  determined? 

124.  What  evidence  is  there  bearing  on  the  question  whether  the  stellar  uni- 
verse is  an  orderly  system  ? 


QUERIES    FOR    USE    IN    REVIEWS    AND    EXAMINATIONS.       319 

125.  %Phat  is  the  method  of  naming  double  stars  ? 

126.  State  the  distinction  between  physical  and  optical  doubles. 

127.  How  does  the  spectroscope  show  that  some  stars  are  binaries,  when  sim- 
ple visual  observations  with  a  telescope  would  never  reveal  the  fact  ? 

1 28.  Tell  the  story  of  the  discovery  of  the  companion  of  Sirius. 

129.  Describe  two  multiple  stars. 

130.  Define  a  variable  star,  and  a  periodic  variable. 

131.  State  the  five  classes  of  variable  stars. 

132.  Give  an  account  of  Tycho's  temporary  star. 

133.  Describe  the  variations  of  Algol,  and  their  cause. 

134.  Tell  the  story  of  Nova  Aurigae. 

135.  How  are  variables  observed  by  astronomers  ? 

136.  State  the  supposed  causes  of  stellar  variability. 

137.  Tell  the  different  forms  of  nebulae. 

138.  What  is  the  law  of  distribution  of  the  nebulae  over  the  face,  of  the  sky  ?  • 

139.  What  kinds  of  spectra  do  nebulae  give  ? 

140.  Describe  the  great  nebula  in  Orion,  or  that  in  Andromeda. 

141.  What  are  the  Magellanic  Clouds? 

142.  What  is  Professor  Holden's  theory  of  the  real  form  of  spiral  nebulae  ? 

143.  State  the  nebular  hypothesis  according  to  Laplace. 

144.  What  modifications  of  Laplace's  theory  have  been  made  ? 

145.  Give  the  testimony  of  the  nebulae  and  of  the  stars  to  the  truth  of  the 
nebular  hypothesis. 

146.  State  the  testimony  to  the  truth  of  the  nebular  hypothesis  given  by  the 
motions  of  the  planets. 

147.  What  is  the  testimony  of  astronomy  as  to  the  future  of  the  visible  uni- 
verse ? 

148.  Tell  how  to  find  the  northern  constellations  on  any  evening  by  the  aid 
of  Map  I. 

149.  Tell  how  to  find  the  southern  constellations  on  any  evening  by  the  aid  of 
Maps  II.  to  V. 

150.  Give  some  hints  useful  for  learning  and  fixing  in  mind  the  constellations. 


32O  DESCRIPTIVE    ASTRONOMY. 


APPENDIX   VII. 

476.     LIST    OF    REFERENCE    BOOKS. 

THE  following  list  of  books  upon  Descriptive  Astronomy  is  given  to  aid  in 
the  formation  of  a  reference  library.  With  such  a  wealth  of  good  material 
to  choose  from,  one  ought  not  to  go  far  astray.  A  generous  selection  of  such 
books  would  be  found  very  helpful. 

BALL.     Atlas  of  Astronomy.     D.  Appleton  &  Co.,  Publishers.     $4.00. 

There  are  72  plates,  34  of  which  are  devoted  to  star  maps,  on  which  all  stars 
down  to  the  sixth  magnitude  inclusive  are  shown  with  great  distinctness.  The 
very  complete  index  map  of  the  moon  occupies  several  plates.  Directions  are 
given  for  locating  the  planets  among  the  stars,  up  to  1902.  The  list  of  select  tele- 
scopic objects  contains  exceptionally  full  descriptions  of  them.  Many  unique 
features  commend  the  Atlas  strongly  to  students  and  amateur  observers. 

BALL.     Great  Astronomers.     Isbister  &  Co.,  Publishers,     pp.  372.     73.  6d. 
In  this  sketchy  book  are  pen  pictures  of  18  astronomers  from  Ptolemy  on- 
ward.    There  is  much  chatty  information,  together  with  numerous  illustrations  of 
these  famous  men  and  their  observatories. 

BALL.      In   Starry  Realms.     J.    B.   Lippincott   Co.,    Importers,      pp.  364. 
$2.50. 

A  series  of  finely  written  essays  on  interesting  matters  pertaining  to  the 
heavenly  bodies.  They  are  suited  for  supplementary  reading  in  connection  with 
any  text-book  on  elementary  astronomy.  There  are  two  non-astronomical  chap- 
ters, one  devoted  to  the  eruption  of  Krakatoa  in  August,  1883,  and  the  other  to 
the  relation  of  Darwinism  to  various  sciences. 

BALL.     In   the    High   Heavens.      J.   B.  Lippincott    Co.,    Publishers.      pp. 

383.     $2.50. 
A  readable  book  on  various  astronomical  topics  of  interest. 

BALL.     Starland.     Ginn  &  Co.,  Publishers,     pp.  376.     $1.00. 

A  charming  book  for  boys  and  girls,  and  for  "  children  of  a  larger  growth," 
who  have  a  desire  to  refresh  their  knowledge  of  astronomy.  Gladstone  read  it 
with  pleasure. 

BALL.      The  Story  of  the  Heavens.     Cassell  &  Co.,  Publishers,     pp.  536. 

$5.00. 
A  popular  astronomy,  written  in  a  delightful  style :  the  chapter  on  the  tides  is 


LIST    OF    REFERENCE    BOOKS.  321 

especially  noteworthy :   it  explains  in  a  simple  manner  Prof.   G.  H.   Darwin's 
theory  of  tidal  evolution,  as  illustrated  in  the  case  of  the  moon  and  the  earth. 

BLAKE.     Astronomical  Myths.    Macmillan  &  Co.,  Publishers,    pp.  431.    $2.00. 
This  is  based  on  Flammarion's  "  History  of  the  Heavens."     It  treats  of  the 
beginnings  of  astronomy,  and  of  the  many  theories  held  in  ancient  and  mediaeval 
times  concerning  the  structure  of  the  heavens  and  of  the  earth. 

BOEDDICKER.      The    Milky   Way.      Longmans,   Green   &   Co.,    Publishers. 

$10.00. 

Dr.  Boeddicker  has  prepared  four  plates  of  the  Milky  Way,  each  18  X  23 
inches,  showing  that  wonderful  aggregation  of  suns,  as  it  appears  to  the  keen  eye 
of  a  painstaking  observer.  The  Via  Lactea  is  delineated  from  the  north  pole  to 
10°  of  south  declination.  The  exceeding  complexity  of  its  structure  is  a  revela- 
tion to  one  who  has  never  made  a  careful  study  of  it. 

BREWSTER.      The   Martyrs   of  Science.      Chatto   and   Windus,    Publishers. 

pp.  248.     $1.80. 

Brief  and  interesting  biographies  of  Galileo,  Tycho,  and  Kepler,  by  Sir  David 
Brewster. 

CHAMBERS.      Handbook  of  Descriptive  and  Practical  Astronomy.      Fourth 
Edition.     The  Clarendon  Press,  Oxford,  Publishers.     3  vols.     pp.  1618. 
,i    $14.00. 

A  miniature  encyclopaedia  in  its  field;  valuable  to  amateur  astronomers;  a 
good  reference  book  for  teachers  and  advanced  scholars. 

CHAMBERS.      Pictorial  Astronomy  for  General   Readers.     Whittaker  &  Co., 

Publishers,     pp.  268.     $1.25. 

The  descriptive  matter  is  good,  but  some  of  the  cuts  are  atrociously  executed. 
There  are  lists  of  the  most  interesting  celestial  objects  suitable  for  observation 
with  a  three-inch  telescope. 

CLERKE.     History  of  Astronomy  during  the  Nineteenth  Century.     Adam  and 
Charles  Black,  Publishers.     Third  Edition,  revised  and  enlarged,     pp. 
500.     $4.00. 
An  excellent  work,  written  in  an  interesting  style.     Those  teachers  and  older 

scholars  who  take  special  interest  in  astronomy  will  find  its  perusal  delightful 

and  helpful. 

CLERKE.     The  Herschels  and  Modern  Astronomy.     Macmillan  &  Co.,  Pub- 
lishers,    pp.  224.     $1.25. 

A  delightful  account  of  the  lives  and  scientific  activities  of  Sir  William 
Herschel,  his  devoted  sister  Caroline,  and  his  son  Sir  John.  The  ardent  purposes 
and  high  ideals  of  the  subjects  of  the  sketch  are  well  set  forth. 


322  DESCRIPTIVE    ASTRONOMY. 

CLERKE.     The  System  of  the  Stars.     Longmans,  Green  &  Co.,  Publishers. 

pp.  440.     $7.00. 

This  is  the  most  exhaustive  work  on  the  fixed  stars  in  the  English  language : 
it  is  a  useful  book  of  reference.  Sidereal  astronomy  is  making  rapid  strides, 
which  are  well  described  :  stress  is  laid  upon  the  latest  theories  of  the  construc- 
tion of  the  sidereal  universe. 

COLAS.     Celestial  Planisphere.     Poole  Bros.,  Publishers.     $3.00. 

This  is  one  of  the  best  of  planispheres.  It  consists  of  a  movable  disk,  19 
inches  in  diameter,  attached  by  a  pivot  to  a  heavy  rectangular  piece  of  card- 
board which  measures  18^X23  inches.  Nearly  all  stars  visible  to  the  naked 
eye,  down  to  50°  of  south  declination,  and  the  chief  nebulae,  are  depicted  on  it. 
It  can,  like  all  planispheres,  be  adjusted  and  held  in  such  a  way  as  to  show  the 
face  of  the  sky  at  any  moment ;  the  time  when  any  star  rises,  sets,  or  culminates 
on  a  given  day,  can  be  ascertained  from  it.  It  is  accompanied  by  a  celestial 
handbook  of  1 10  pages,  in  which,  after  a  few  pages  of  definitions,  detailed  de- 
scriptions of  the  constellations  are  given  ;  the  principal  objects  of  interest  in  each 
are  mentioned.  The  price  of  the  handbook  is  $2.00. 

COLAS.     The  Moon,  a  Map.     Poole  Bros.,  Publishers. 

The  map  is  20  inches  in  diameter,  and  is  printed  in  colors  :  an  index  pamphlet 
of  24  pages  by  Prof.  W.  W.  Payne  accompanies  it.  Extremely  satisfactory. 

DENNING.     Telescopic  Work  for  Starlight  Evenings.      Taylor  and  Francis, 

Publishers,     pp.  361.     $2.00. 

The  first  three  chapters  are  devoted  to  the  telescope :  the  methods  of  testing 
and  handling  it  are  explained.  The  relative  merits  of  refractors  and  reflectors 
are  set  forth.  The  remaining  chapters  are  filled  with  descriptive  matter  about 
the  heavenly  bodies.  Meteors  and  meteoric  observations  are  treated  quite  fully, 
the  author  being  a  specialist  in  that  line  of  work. 

DREYER.     Tycho  Brahe,  a  Picture  of  Scientific  Life  and  Work  in  the  Six- 
teenth Century.    Adam  and  Charles  Black,  Publishers,    pp.  405.    $3.50. 
A  thoroughly  reliable  and  readable  account  of  the  life  and  scientific  surround- 
ings of  one  of  the  greatest  of  astronomers. 

ELGER.     The  Moon.     George  Philip  and  Son,  Publishers,     pp.  173.     55. 

This  work  is  devoted  to  a  description  of  the  craters,  seas,  etc.,  on  the 
lunar  landscape,  and  contains  excellent  maps.  The  best  medium-priced  lunar 
handbook. 

FROST.  Astronomical  Spectroscopy.  Translated  from  the  German  of  Dr. 
J.  Scheiner,  and  revised  with  the  author's  sanction.  Ginn  &  Co.,  Pub- 
lishers, pp.  450.  $5.00. 

The  most  practical  methods  of  spectroscopic  observations  are  set  forth  in  de- 
tail, and  the  knowledge  thus  far  gained  by  means  of  astronomical  spectroscopy 


LIST    OF    REFERENCE    BOOKS.  323 

is  admirably  stated.  The  book  is  fairly  entitled  to  be  called  indispensable  to 
workers  along  spectroscopic  lines.  Teachers  will  find  it  useful  as  a  work 
of  reference. 

GORE.     Flammarion's  Popular  Astronomy.     Chatto  and  Windus,  Publishers. 

pp.  679.     1 6  shillings. 

This  is  a  translation  from  the  French,  the  original  having  reached  a  sale  of 
over  100,000  copies.  The  book  is  finely  illustrated,  and  very  popular  in  style. 

GORE.  J.  E.     The  Scenery  of  the  Heavens.     Roper  and  Drowley,  Publishers. 

pp.32o.     $4.00. 

This  work  contains  a  general  account  of  the  heavenly  bodies,  together  with 
lists  and  descriptions  of  the  most  interesting  double  stars,  nebulae,  and  vari- 
able stars. 

GORE,  J.  E.     The  Visible  Universe.     Macmillan  &  Co.,  Publishers,    pp.  340. 

$3-75- 

This  work  deals  with  the  different  theories  of  solar  and  stellar  evolution,  the 
ether,  the  constitution  of  matter,  and  the  theories  of  the  shape  of  the  visible  uni- 
verse, large  space  being  given  to  the  last  subject.  The  elegance  of  the  illustra- 
tions befits  the  excellence  of  the  text. 

GORE,  J.   H.     Geodesy.     Houghton,  MifHin  &  Co.,  Publishers,     pp.  218. 

$1.25. 

The  author  has  given  an  historical  sketch  ot  the  various  important  attempts 
to  measure  the  magnitude  and  determine  the  form  of  the  earth,  from  the 
earliest  times. 

KIRKWOOD.      The  Asteroids.     J.   B.   Lippincott   Co.,    Publishers.      pp.  60. 

$0.50. 

In  addition  to  general  descriptive  matter,  the  author  makes  a  special  study  of 
the  distribution  of  the  orbits  of  asteroids,  giving  reasons  for  the  gaps  which  are 
found  in  them. 

KLEIN.     Star  Atlas.     E.  &  J.  B.  Young  &  Co.,  Publishers,     pp.  72,  aside 
from  maps  and  plates.     $2.00. 

The  maps  are  twelve  in  number,  each  measuring  12  X  9  inches  :  they  contain 
all  stars,  from  the  sixth  magnitude  upward,  between  the  north  pole  and  30°  of 
south  declination.  There  is  a  6o-page  list  of  interesting  telescopic  objects,  with 
a  good  description  of  each  one.  This  is  probably  the  best  low-priced  atlas  in 
the  English  language. 

LANGLEY.      The   New   Astronomy.      Houghton,  Mifflin  &  Co.,  Publishers. 

pp.  260.     $5.00. 

The  term  "  new  astronomy "  is  used  chiefly  with  reference  to  spectroscopic 
and  photographic  work.  The  book  is  very  finely  illustrated :  the  author's  style  is 


324  DESCRIPTIVE    ASTRONOMY. 

so  elegant  that  the  reader's  attention  is  closely  held  throughout.  It  would 
be  hard  to  find  an  astronomical  work  more  attractively  written,  or  better 
illustrated. 

LOCKYER.     The  Dawn  of  Astronomy.     Macmillan  &  Co.,  Publishers,     pp. 

432.     $5.00. 

This  work  contains  a  study  of  the  mythology  and  temple  worship  of  the 
ancient  Egyptians,  with  special  reference  to  their  astronomical  bearings.  The 
book  is  elaborately  illustrated,  and  is  suitable  for  reference,  rather  than  for  gen- 
eral reading. 

LOCKYER.     The  Meteoritic   Hypothesis.     Macmillan  &  Co.,  Publishers,     pp. 
560.     $5.25. 

The  sub-title  is  "  A  Statement  of  the  Results  of  a  Spectroscopic  Inquiry  into 
the  Origin  of  Cosmical  Systems."  The  book  sets  forth  the  theory  that  all 
celestial  bodies  are  composed  of  meteors,  more  or  less  thickly  crowded  together, 
and  gives  in  detail  the  observations  and  experiments  on  which  the  theory  is 
based.  The  illustrations  are  fine :  some  portions  of  the  text  are  of  interest  to 
non-astronomical  readers. 

LOWELL.     Mars.    Hough  ton,  Miffh'n  &  Co.,  Publishers,     pp.  217.     $2.50. 

A  popular  account  of  Mars,  with  special  reference  to  the  question  of  the 
existence  of  intelligent  beings  on  its  surface :  elegantly  illustrated  by  full  page 
plates. 

MITCHEL.     Ormsby  Macknight  Mitchel,  Astronomer  and  General.     Hough- 
ton,  Mifflin  &  Co.,  Publishers,     pp.  392.     $2.00. 

An  admirable  biography  of  a  remarkable  man,  who  built  the  Cincinnati  Ob- 
servatory, raising  the  necessary  funds  by  a  popular  subscription.  Most  of  the 
subscriptions  came  from  tradesmen  and  mechanics :  many  were  payable  in  com- 
modities or  labor. 

NASMYTH  AND  CARPENTER.     The  Moon.     Scribner  and  Welford.     pp.  213. 

$9.00. 

During  more  than  thirty  years  the  authors  studied  the  moon's  surface  with 
powerful  telescopes ;  they  made  careful  drawings,  and  then  constructed  accurate 
models  of  the  lunar  craters ;  these  were  photographed.  The  book  contains  25 
very  fine  plates. 

NEWCOMB.      Popular  Astronomy.      School  Edition.     Harper  and  Brothers, 

Publishers,     pp.  578.     $1.30. 

An  excellent  presentation  of  the  subject,  written  by  one  of  the  ablest  of 
astronomers. 

PARKER.     Familiar  Talks  on  Astronomy.     A.  C.  McClurg  &  Co.,  Publishers. 

pp.  264.     $1.00. 
These  are  such  talks  as  a  teacher  might  give  to  a  class  in  elementary  as- 


LIST    OF    REFERENCE    BOOKS.  325 

tronomy,  in  addition  to  the  regular  class  work.  The  last  four  chapters  are 
about  time  and  nautical  astronomy,  viewed  from  the  standpoint  of  a  practical 
navigator. 

PROCTOR.     Half  Hours  with  the  Stars.     G.  P.  Putnam's  Sons,  Publishers. 

pp.  38.     $2.00. 

The  book  is  in  quarto  form,  and  contains  twelve  large  maps,  showing  the 
aspect  of  the  heavens  throughout  the  year.  The  text  is  devoted  to  explaining 
how  to  find  the  star  groups.  Very  few  stars  fainter  than  the  fourth  magnitude 
are  shown.  The  work  is  admirably  adapted  to  the  needs  of  a  beginner,  who 
wishes  to  become  familiar  with  the  constellations. 

PROCTOR  AND  RANYARD.  Old  and  New  Astronomy.  Longmans,  Green  & 
Co.,  Publishers,  pp.  824.  $12.00. 

This  volume  is  published  in  the  form  of  a  quarto.  Mr.  Proctor  considered  it 
as,  in  a  sense,  the  summing  up  of  his  numerous  writings  on  astronomy.  It  was 
not  completed  at  the  time  of  his  death,  and  Mr.  Ranyard  supplied  the  portion 
lacking.  The  work  is  a  fitting  crown  to  Mr.  Proctor's  long  service  in  populariz- 
ing astronomy.  Mr.  Ranyard's  careful  editing  and  supplementary  writing  add 
much  to  its  value. 

SERVISS.     Astronomy  with  an  Opera-glass.     D.  Appleton  &  Co.,  Publishers. 

pp.  154.     $1.50. 

Maps  of  the  constellations  are  given,  and  directions  for  finding  them  :  promi- 
nent celestial  objects  are  described;  the  author  gives  directions  for  choosing  a 
good  opera-glass,  and  shows  that  many  pleasurable  views  of  the  heavenly  bodies 
may  be  obtained  by  its  aid. 

THORNTON.  Advanced  Physiography.  Longmans,  Green  &  Co.,  Publishers, 
pp.  338.  $1.40. 

One  of  the  very  best  works  on  this  subject :  it  is  largely  an  elementary  astron- 
omy, but  about  one  fourth  of  the  book  is  devoted  to  atmospheric  and  oceanic 
motions,  magnetism,  and  the  secular  cooling  of  the  earth. 

WEBB.  Celestial  Objects  for  Common  Telescopes.  Fifth  Edition,  revised 
and  greatly  enlarged  by  T.  E.  Espin.  2  vols.  Longmans,  Green  & 
Co.,  Publishers.  $3.50. 

This  is  the  most  complete  and  authoritative  book,  in  its  field,  in  our  language. 
Volume  I.  tells  how  to  use  a  telescope  for  visual,  photographic,  and  spectroscopic 
work,  and  contains  chapters  on  the  sun,  moon,  planets,  comets,  and  meteors. 
Volume  II.  gives  lists  and  full  descriptions  of  the  principal  stars,  clusters,  and 
nebulae  visible  to  us. 

WINCHELL.     World-Life,  or  Comparative  Geology.    S.  C.  Griggs  &  Co.,  Pub- 
lishers,    pp.  642.     $2.50. 
The  work  is  written  from  the  standpoint  of  a  geologist,  and  is  chiefly  devoted 


326  DESCRIPTIVE    ASTRONOMY. 

to  an  elaborate  account  of  the  formation  of  the  different  planets,  in  accordance 
with  the  principles  of  the  nebular  hypothesis.  There  are  also  chapters  on  Plan- 
etary Decay,  The  Habitability  of  Other  Worlds,  and  The  Evolution  of  Cosmo- 
gonic  Doctrine. 

YOUNG.     The  Sun.     D.  Appleton  &  Co.,  Publishers,     pp.  363.     $2.00. 

The  best  work  on  this  subject,  in  English :  suitable  for  reference  and  for  col- 
lateral reading.  The  author  is  one  of  the  most  distinguished  students  of  the 
sun. 

YOUNG.     General  Astronomy.     Ginn  &  Co.,  Publishers,     pp.  551.     $2.50. 

A  text-book  for  advanced  collegiate  students,  replete  with  accurate  informa- 
tion :  easily  the  first  of  its  kind. 


INDEX. 


All  numerical  references  are  to  Sections, 


Abbe,  Professor,  39. 

Aberration  of  light,  116. 

Achromatic  Object-glass,  39. 

Achromatism,  almost  perfect,  39. 

Adams,  J.  C.,  computes  orbit  of  Neptune,  271. 

Aerolite.     (See  Meteorites.) 

Age  of  the  sun,  87. 

Algol,  diameter  of,  348 ;  variations  of,  379 ;  position,  449. 

Alpha  Centauri,  349,  350. 

Alphabet  of  the  Greek  language,  405. 

Altitude  defined,  121. 

Anderson,  Thos.  D.,  discovers  Nova  Aurigae,  381. 

Andromeda,  nebula  in,  389,  398,  407 ;  constellation,  407. 

Andromedes,  330. 

Annular  eclipses,  175,  176;  nebulae,  385. 

Aphelion,  explained,  96. 

Apparent  Motion,  of  the  stars  daily,  13;  of  the  celestial  sphere,  17;  of  the 

sun,  125. 
Aquarius,  408. 
Aquila,  409. 
Argo  Navis,  410. 
Aries,  a  sign  of  the  zodiac,  100;  the  sun  enters,  112;  non-coincidence  of  sign 

and  constellation,  112;  the  constellation,  411. 
Aspects  of  the  planets,  188. 
Asteroids,  discovery,  224-227 ;    orbits,  distances,   periods,   228 ;    designations, 

229;    number    and    sizes,    230;     atmosphere    and    surface    gravity,    231  ; 

origin,    232. 

Astrsea  discovered,  226. 
Astronomical  Constants,  471. 
Astronomy,  landmarks  in  the  history  of,  473. 

Atmosphere  of  the  moon,  162  and  164;  of  the  planets  (see  each  planet). 
Attraction  of  gravitation,  183,  368. 
Auriga,  412. 
Azimuth,  defined,  121. 


328  INDEX. 

Bailey,  S.  I.,  discovers  variables  in  clusters,  373. 

Ball,  Sir  Robert,  accounts  for  the  rarity  of  the  atmosphere  of  Mars,  217. 

Barnard,  E.  E.,  drawing  of  Mars,  213;  observes  changes  on  Mars,  215  ;  ob- 
serves Jupiter's  moons,  249;  discovers  the  fifth  satellite  of  Jupiter,  251; 
observes  a  moon  of  Saturn  in  the  shadow  of  the  dark  ring,  260  ;  discovers  a 
comet  by  photography,  280  ;  photographs  Brooks's  comet,  307  ;  observes  the 
Gegenschein,  324,  note;  first  photographed  the  star  clouds  of  the  Milky 
Way,  338. 

Berliner  Jahrbuch,  228. 

Beta  Lyrse,  378, 

Bethlehem,  star  of,  201. 

Biela  discovers  a  comet,  303. 

Bielids,  330. 

Binary  Stars,  368 ;  spectroscopic,  369. 

Black  Drop,  204. 

Bode's  Law,  223. 

Books  for  reference,  476. 

Bootes,  413. 

Brashear,  J.  A.,  optician,  39. 

Bredichin,  investigates  the  forms  of  comets'  tails,  295. 

Brooks,  Wm.  R.,  discovers  comet  c  1893,  307. 

Bruce  photographic  telescope,  277,  345. 

Burnham,  S.  W.,  366. 

Caesar,  Julius,  his  calendar,  114. 

Calendar,  the  Julian,  114;  the  Gregorian,  115. 

Camelopardus,  414. 

Campbell,  W.  W.,  observes  Jupiter's  moons  with  the  Lick  telescope,  249;  also- 

observes  Nova  Aurigae,  381. 
Canals  of  Mars,  218,  219. 
Cancer,  415. 
Canes  Venatici,  416. 
Canis  Major,  417. 
Canis  Minor,  418. 
Capricornus,  419. 
Capture  of  comets,  287. 
Carrington,  observation  of  the  sun,  60. 
Cassini  discovers  the  main  division  of  Saturn's  rings,  258. 
Cassiopeia,  1 1 ,  406,  420. 
Catalogues  of  stars,  344. 

Celestial  Sphere,  defined,  15,  16;  daily  motion  of,  17. 
Centaur  us,  421. 
Cepheus,  422. 
Ceres  discovered,  224,  225. 
Cetus,  423. 


INDEX.  329 

Chandler,  S.  C.,  catalogue  of  variables,  373 ;  period  of  Algol,  379 ;  variation  of 

latitude,  95. 

Changes  on  the  moon,  161. 
Chinese  record  of  a  meteor,  314. 
Chromosphere,  described,  76. 
Chronograph,  139,  140. 
Chronometer,  used  on  shipboard,  143. 
Civil  Day,  129. 
Clairaut,  investigates  the  orbit  of  Halley's  comet,  301. 

Clark,  Alvan,  39 ;  his  son,  Alvan  G.,  discovers  the  companion  of  Sirius,  370. 
Classification,  of  the  planets,  187;   of  stellar  spectra,  353;   of  variable  stars, 

374- 

Clocks,  135;  their  errors  determined,  138. 

Clusters  of  stars,  347  ;  list  of,  469. 

Colored  stars,  list  of,  469. 

Columba,  424. 

Coma  of  a  comet,  283  ;  first  appearance  of,  289. 

Coma  Berenices,  425. 

Comet,  of  1861,  5,  300;  of  1843,  291;  1889  V  (Brooks),  292;  Halley's,  299, 
301;  of  1528,  299;  Biela's,  300,  303,  306,  330,  332;  Encke's,  302;  Holmes's, 
303,  306;  of  1882,  304;  Swift's,  of  1892,  305;  c  1893  (Brooks),  307;  Tern- 
pel's,  332. 

Comets,  derivation,  5;  in  general,  279;  discovery,  280;  number  and  designation, 
281 ;  brightness  and  visibility,  282;  parts  of,  283  ;  forms  of  orbits,  284;  sig- 
nificance of  forms  of  orbits,  285  ;  groups,  286 ;  planetary  families,  287 ; 
changes  in  orbits,  288  ;  changes  of  appearance,  289  ;  jets  and  envelopes,  290; 
tails,  291  ;  companion  comets,  292 ;  constitution,  293 ;  evolution  of  the  tail, 
294;  types  of  tails,  295  ;  mass  and  density,  296 ;  light  and  spectra,  297;  fate, 
298 ;  superstitions,  299 ;  collisions,  300. 

Common,  A.  A.,  his  large  reflector,  43. 

Conic  Sections,  284. 

Conjunction  of  planets,  188. 

Constants,  list  of,  471. 

Constellations,  defined,  i ;  names  of,  9;  how  to  find,  10,  n  ;  history  of,  340. 

Constitution  of  the  sun,  88. 

Contraction  of  the  sun,  86. 

Copernicus  publishes  his  great  work  in  1543,  473. 

Corona,  appearance  of,  80 ;  Schaeberle's  theory  of,  81  ;  nature  of,  82. 

Corona  Borealis,  426. 

Coronium,  82. 

Corvus,  427. 

Crater,  428. 

Craters  on  the  moon,  157,  158. 

Crescent  moon,  150. 

Cygnus,  429. 


330  INDEX. 

Data,  planetary,  472 ;  historical,  473. 

Date,  change  of,  133. 

Day,  length  of,  103-105  ;  mean  and  apparent  solar,  125-127;  sidereal,  128;  civil 

and  astronomical,  129. 
Declination,  defined,  122. 
Deimos,  221. 
Delphinus,  430. 
Designation,  of  asteroids,  229;  of  comets,  281;  of  stars,  341;  of  double  stars, 

366  ;  of  variable  stars,  373. 

Dimensions,  of  comets'  tails,  291 ;  of  stars,  348. 

Directions  for  finding  an  object  by  means  of  an  equatorial  telescope,  468. 
Disk  of  a  star,  348. 

Dispersion  of  light,  37;  corrected,  38. 
Displacement  of  spectral  lines,  360. 
Distance,  of  the  sun,  49;  determined  by  parallax,  123,  351  ;  of  the  moon,  145  ; 

of  the  stars,  349 ;  of  the  nebulae,  386 ;  of  each  planet,  472. 
Distribution,  of  the  stars,  346 ;  of  the  nebulae,  386. 
Double  Stars,  appearance,  364,  365 ;    number  and  nomenclature,  366 ;   optical, 

367;  physical,  368;  spectroscopic,  369;  Sirius,  370;  planetary  systems,  371  ; 

evolution  of,  397  ;  list  of,  469. 
Draco,  431. 

Draper,  J.  W.,  in  1840,  first  photographed  the  moon,  473. 
Dumb-bell  Nebula,  391. 
Duration  of  human  life  on  the  earth,  87  ;  of  eclipses,  179. 

Earth,  dimensions  and  shape,  90 ;  diameter,  how  measured,  92 ;  latitude  and 
longitude  on  it,  93,  94 ;  variation  of  latitude,  95  ;  the  orbit,  96 ;  the  ecliptic, 
97;  the  equinoxes,  98;  the  solstices,  99;  zodiac,  100;  length  of  the  day, 
103-105;  midnight  sun,  106 ;  seasons,  107,108;  equatorial  ring,  109;  pre- 
cession explained,  109-111;  effects  of  precession,  112;  different  kinds  of 
years,  113;  Julian  calendar,  114;  Gregorian  calendar,  115;  aberration  of 
light,  116;  atmospheric  refraction,  117;  twilight,  118. 

Earth  Shine  on  the  moon,  151. 

Eclipses,  the  one  of  April  16,  1893,  81  ;  of  the  moon,  170-172;  cause  of  solar, 
173;  varieties  of  solar,  175;  partial  and  annular  solar,  176;  total  solar,  177, 
178;  duration  and  number  of,  179;  of  Jupiter's  satellites,  246. 

Ecliptic,  defined,  97;  fixity  of,  102. 

Ellipse,  described,  96  ;  a  conic  section,  284;  changed  into  a  parabola  or  hyper- 
bola, 288. 

Elongation  of  a  planet,  188. 

Encke  discovers  a  division  in  Saturn's  rings,  258. 

Envelopes  of  comets,  290. 

Ephemerides  of  asteroids,  228. 

Epsilon  Lyrae,  372. 

Equator,  Celestial,  defined,  20;  fixity  of,  102. 


INDEX.  331 

Equatorial  Mounting,  explained,  44-46. 

Equinoxes,  defined,  98;  precession  of,  explained,  109-112. 

Equuleus,  432. 

Eridanus,  433. 

Essays,  topics  for,  474. 

Evening  Star,  defined,  191  ;  Venus,  201. 

Evolution  of  double  stars,  397. 

Eyepieces,  their  action,  33;  achromatic,  40;  negative  and  positive,  40. 

Examinations,  queries  for,  475. 

Faculce,  54. 

Families  of  comets,  287. 

Galaxy,  338. 

Galileo,  discovers  the  phases  of  Venus,  205 ;  thinks  Saturn  triform,  257 ;  notices 

the  vibrations  of  a  pendulum  in   1583,473;  makes  a  telescope  and  various 

discoveries  with  it  in  1609-1613,  473. 
Galle  discovers  Neptune,  271. 
Gauss  computes  the  orbit  of  Ceres,  225. 
Gegenschein,  334. 
Gemini,  434. 
Gibbous  moon,  150. 

Gravitation,  law  of,  183  ;  universal,  368. 
Great  Circles  defined,  [20. 
Great  Dipper,  20,  359,  369. 

Greek  Alphabet,  used  in  naming  stars,  341  ;  given,  405. 
Gregorian  Calendar,  115. 
Groups  of  stars,  359. 

Habitability  of  Mars,  222. 

Hale,  Geo.  E.,  observes  a  solar  disturbance,  61. 

Hall,  A.,  discovers  the  moons  of  Mars,  221  ;  monograph  on  the  moons  of  Mars, 
222,  note;  determines  rotation  of  Saturn,  254. 

Halley,  his  method  of  observing  a  transit  of  Venus,  204;  his  comet,  299,  301  ; 
in  1705  predicted  its  return,  473;  in  1731  invented  the  sextant,  473. 

Heat,  of  the  sun,  84;  produced  by  contraction,  86;  of  the  moon,  165. 

Hercules,  435. 

Herschel,  Caroline,  265. 

Herschel,  Sir  John,  makes  a  prediction  about  Neptune,  271;  star-gauges 
of,  346. 

Herschel,  Sir  William,  discovers  Uranus,  265;  star-gauges,  346;  in  1803  pub- 
lishes his  discovery  of  binary  stars,  473. 

Hesperus,  201. 

History  of  astronomy,  473. 

Hodgson,  observation  of  the  sun,  60. 


332  INDEX. 

Holden,  E.  S.,  opinion  of  the  polar  caps  of  Mars,  213  ;  finds  motion  in  the  trifid 
nebula,  387 ;  investigates  spiral  nebulae,  392. 

Holmes  discovers  a  comet,  306. 

Horizon  defined,  21,  121. 

Horizontal  parallax,  123. 

Hour  Angle,  defined,  122;  of  a  star  at  any  instant,  466. 

Hour  Circles  defined,  122. 

Huyghens,  discovers  the  rings  of  Saturn  in  1665,  257,  473  ;  made  the  first  pen- 
dulum clock  in  1656,  473. 

Hydra,  436. 

Hyperbola,  a  conic  section,  284 ;  a  comet's  orbit,  285. 

Image,  formation  of,  32. 

Inertia,  184. 

Inti  a-Mercurial  planets,  1 78. 

Japetus,  263. 

Jena  glass,  39. 

Jets  from  comets,  290. 

Josephus  mentions  a  comet,  299. 

Julian  Calendar,  1 14. 

Juno  discovered,  226. 

Jupiter,  occulted  by  the  moon,  162  ;  may  have  disrupted  the  asteroid  ring,  232  ; 
distance  and  diameter,  235;  revolution  and  rotation,  236;  appearance,  237- 
241;  satellites  visible  with  the  naked  eye,  237;  belts,  238,  239;  great  red 
spot,  240;  other  spots,  239,  241  ;  atmosphere  and  spectrum,  242;  light  and 
heat,  243 ;  physical  condition,  244 ;  the  major  satellites,  245  ;  eclipses,  occul- 
tations,  and  transits  of  the  satellites,  246-248 ;  markings  and  rotation  of  the 
satellites,  249,  250;  the  fifth  satellite,  251 ;  observations  of  the  moons  give 
the  velocity  of  light,  252  ;  gathers  a  family  of  comets,  287  ,  disturbs  Brooks's 
comet,  292. 

Kapteyn,  J.  C.,  investigates  the  form  of  the  stellar  system,  357. 

Keeler,  J.  E.,  discovers  a  division  in  Saturn's  rings,  258  ;  determines  their  struc- 
ture, 261  ;  determines  the  motion  of  the  nebula  in  Orion,  390. 

Kepler,  his  speculations  on  lunar  craters,  157  ;  his  laws,  185  ;  guesses  the  number 
of  moons  of  the  planets,  222,  note;  his  opinion  about  comets,  281 ;  publishes 
his  first  and  second  laws  in  1609,  473  ;  discovers  his  third  law  in  1618,  473. 

Kirchhoff's  Laws,  73. 

Krakatoa,  eruption  of,  324. 

Lacerta,  437. 

Landmarks  in  the  history  of  astronomy,  473. 

Langley,  S.  P.,  estimate  of  work  done  by  the  sun's  heat,  84 ;  observations  of 

Mars,  215. 
Laplace  suggests  a  name  for  Uranus,  265  ;  his  nebular  hypothesis,  395,  396. 


INDEX.  333 

Latitude,  of  terrestrial  points,  93,  94 ;  variation  of,  95  ;  method  of  determining, 

141  ;  of  a  ship,  143. 
Leap  Year,  115. 
Lenses,  30. 
Leo,  438. 
Leo  Minor,  439. 
Leonids,  327-329. 
Lepus,  440. 

Leverrier  computes  the  orbit  of  Neptune,  271. 
Libra,  441. 

Librations  of  the  moon,  149. 
Lick  telescope,  48, 
Life  on  the  moon,  167. 

Light,  of  the  sun,  83  ;  velocity  of,  83  ;  of  the  moon,  165. 
Light  Ratio,  343. 
Light  Year,  349. 
Lists  of  telescopic  objects,  469. 
Local  Time,  130. 

Lockyer,  J.  N.,  his  theory  of  variables,  382. 
Longitude,  of  terrestrial  points,  93,  94 ;  determined  by  telegraph,  142  ;  of  a  ship, 

143- 

Lowell,  Percival,  219. 
Lowell  Observatory,  218,  221. 
Lupus,  442. 
Lynx,  443. 
Lyra,  444. 

Magellanic  Clouds,  393. 

Magnetic  Storms,  65-68. 

Magnifying  Power  of  eyepieces,  36. 

Magnitudes  of  stars,  I,  8,  342,  343. 

Maps  explained,  8. 

Mars,  distance  and  diameter,  209 ;    revolution  and  rotation,  210  ;  appearance,  211, 

212;  phases,  212;  polar  caps,  213  ;  seas,  214;  continents  and  islands,  215  ; 

clouds,  216;  atmosphere,  217 ;  canals,  218,219;  colors,  220;  satellites,  221; 

habitability,  222;   Hall's  monograph  on  the  moons,  222,  note. 
Mean  Solar  Time,  130. 
Mercury,  distance  and  diameter,  195  ;  revolution  and  rotation,  196;  transits,  197-, 

appearance  and  phases,  198,199;  physical  condition,  200 ;  perturbs  Encke's 

comet,  302;  transit  first  observed  in  1631,  473. 
Meridian,  terrestrial,  denned,  94;  celestial,  denned,  121. 

Meridian  Circle,  described,  136 ;  used  to  determine  time,  137-138  ;  used  to  deter- 
mine latitude,  141. 
Meteorites,  past  appearances,  309 ;    Ensisheim  meteorite,  310;  Kiowa  County, 

Kansas,  meteorite.  312  ;  in  flight,  313  ;  path  and  velocity,  314  ;  light  and  heat, 


334  INDEX. 

315,316;  meteoric  stones,  317;  meteoric  iron,  318;  elements  found  in,  319; 
Canyon  Diablo,  319;  origin,  320;  observation  of,  321. 

Meteors,  defined,  6  ;  two  classes,  308  ;  a  detonating,  31 1  ;  path  and  velocity,  314; 
light  and  heat,  315,  316;  meteoric  stones,  317  ;  meteoric  iron,  318;  elements 
found  in,  319;  origin,  320;  observation  of,  321. 

Midnight  Sun,  106. 

Milky  Way,  3^8  ;  tree-like  structures  in,  339  ;  shape,  356. 

Minor  Planets.    (See  Asteroids.) 

Mira  Ceti,  376. 

Mizar,  369. 

Monoceros,  445. 

Month,  sidereal  and  synodic,  146. 

Moon,  distance,  145;  diameter,  145;  orbit,  145  ;  nodes,  145  ;  periods,  sidereal 
and  synodic,  146;  meridian  passage,  147;  rotation,  148;  librations,  149; 
phases,  150;  earth  shine,  151 ;  occultations,  152  ;  appearance  to  the  naked  eye, 
153;  telescopic  appearance,  154;  topography,  155;  the  plains,  156;  craters, 
lS7i  I5^'i  mountains,  159;  rills,  clefts,  and  rays,  160;  changes,  161 ;  atmos- 
phere, 162;  spectrum,  162;  water,  163,  164;  light  and  heat,  165;  tempera- 
ture, 1 66;  life,  167;  effect  on  the  weather,  1 68  t  worth  to  man,  169;  eclipses, 
170-172;  mountains  visible  in  a  solar  eclipse,  176;  duration  and  number 
of  eclipses,  1 79 ;  origin  of  its  features,  400. 

Morning  Star,  defined,  191  ;    Venus,  201. 

Motion,  daily,  of  the  heavens,  13,  17. 

Mountains,  lunar,  159;  terrestrial,  400. 

Mount  Hamilton,  clouds  visible  there,  238. 

Mounting,  equatorial,  44-46. 

Multiple  Stars,  372. 

Nadir  defined,  23. 

Nebulas,  defined,  7 ;  various  forms,  385  ;  number,  distance,  and  grouping,  386 ; 
sizes  and  changes  of  appearance,  387 ;  spectra,  388  ;  nebula  in  Andromeda, 
389  ;  nebula  in  Orion,  390  ;  other  notable,  391 ;  real  form  of  spiral,  392  ;  Ma- 
gellanic  Clouds,  393  ;  the  nebular  hypothesis,  394-404 ;  list  of,  469. 

Nebular  Hypothesis,  general  statement,  394 ;  Laplace's  theory  and  its  modifica- 
tions, 395,  396 ;  evolution  of  double  stars,  397  ;  testimony  of  the  nebulae,, 
stars,  earth^  moon,  planetary  systems,  and  sun,  398-402 ;  its  probable  truth, 
403  ;  future  of  the  universe,  404. 

Negative  Eyepieces,  40. 

Neptune,  does  not  conform  to  Bode's  law,  223:  discovery,  271  ;  distance  and 
diameter,  272 ;  revolution  and  rotation,  273  ;  appearance,  274  ;  satellite,  275  ; 
physical  condition,  276 ;  captures  comets.  287. 

Newton,  Sir  Isaac,  law  of  gravitation,  183  ;  published  the  Principia  in  1687,  473.- 

Newtonian  telescope,  41. 

Nodes  of  the  moon's  orbit,  145. 

Nordenskiold  finds  supposed  dust  of  shooting  stars,  324. 


INDEX.  335 

North  Polar  Distance  defined,  122. 
Nova  Aurigee,  381. 

Nucleus  of  a  comet,  283  ;  changes  of,  289. 

Number,  of  eclipses  in  a  year,  179;    of  asteroids,  230;  of  comets,  281 ;  of  shoot- 
ing stars,  322 ;  of  fixed  stars,  336;  of  double  stars,  366;  of  nebulae,  386. 

Object-glass,  use  of  a  large  one,  35 ;  achromatic,  39. 

Oblate  Spheroid,  90. 

Obliquity  of  the  ecliptic  defined,  97. 

Occultations,  by  the  moon,  152  ;  of  Jupiter's  satellites,  247. 

Omicron  Ceti,  376. 

Ophiuchus,  446. 

Opposition  of  a  planet,  188. 

Orbits  of  the  planets,  181. 

Origin,  of  the  asteroids,  232  ;  of  meteors,  320 

Orion,  nebula  in,  390,  447  ;  constellation,  447. 

/ 
Pallas  discovered,  226. 

Parabola,  a  conic  section,  284;  a  comet's  orbit,  285. 

Parallax,  defined,   123;  horizontal,  123;  equatorial  horizontal,  123;  stellar,  350, 

351 ;  of  nebulae,  386. 
Pare  describes  a  comet,  299. 
Pegasus,  448. 

Pendulum,  compensation  of,  135. 
Penumbra,  of  a  sun  spot,  55  ;  of  a  shadow,  170. 
Perihelion  explained,  96. 
Periodic, Comet  defined,  286. 

Periods,  sidereal  and  synodic,  of  the  moon,  146;  of  planets,  192. 
Perseids,  326. 
Perseus,  449. 
Perturbations,  186. 
Phases  of  the  moon,  1 50. 
Phobos,  221. 
Phosphorus,  201. 

Photographic  Congress  in  1887,  473. 
Photographs,  of  faculae,  54 ;  of  a  solar  disturbance,  61  ;  of  the  corona,  81  ;  of 

the  moon,  161 ;   of  asteroids,  227;   of  an  ultra-Neptunian  planet,  277;   of 

comets,  280,  305,  307  ;   of  shooting  stars,  333 ;   of  the  Milky  Way,  338 ; 

of  stars,  345  ;  of  clusters,  347;  of  nebulae,  389-391. 
Photosphere  of  the  sun,  52. 
Piazzi  discovers  the  first  asteroid,  224. 
Pickering,  E.  C.,  estimates  diameters  of  the  moons  of  Mars,  221  ;   plans  star 

charting,  345  ;    views  of  stellar  spectra,  354 ;    announces   the  duplicity  of 

Mizar,  369. 
Pickering,  W.  H.,  opinion  of  the  water  area  of   Mars,  214;    observations  of 


336  INDEX. 

the  canals  of  Mars,  218;  observations  of  Jupiter's  belts,  239;  observes  Jupi- 
ter's satellites,  250. 

Pisces,  450. 

Piscis  Australis,  45 1 . 

Planetary  Data,  472. 

Planetary  nebulae,  385,  398. 

Planets,  defined,  2;  orbits,  181 ;  motion,  182-186;  two  classes,  187 ;  aspects, 
188;  apparent  movements,  189-190  ;  evening  and  morning  stars,^i 91 ;  peri- 
ods, sidereal  and  synodic,  192;  two  groups  of,  194;  Mercury,  195-200; 
Venus,  201-208 ;  Mars,  209-222 ;  minor  planets,  223-232;  Jupiter,  235-252; 
Saturn,  253-264;  Uranus,  265-270;  Neptune,  271-276;  beyond  Neptune, 
277;  testimony  for  the  nebular  hypothesis,  401  ;  data  concerning,  472. 

Pleiades,  347;  proper  motions  of,  359;  nebulous  matter  in,  386. 

Plumb-line,  direction  of,  91. 

Pointers,  18. 

Pole,  location  of  the  north  celestial,  18  ;  fixity  of,  101 ;  precessional  motion  of,  112. 

Positive  Eyepieces,  40. 

Freesepe,  347. 

Precession,  explained,  109-111 ;  effects  of,  112. 

Prime  Vertical  defined,  121. 

Prominences,  Solar,  quiescent  and  eruptive,  77 ;  seen  with  the  spectroscope, 
78 ;  associated  with  magnetic  storms,  79. 

Proper  Motion  of  the  stars,  358,  359. 

Ptolemy,  revises  the  scheme  of  constellations,  340 ;  his  work,  473. 

Pupin,  M.  I.,  electrical  discharges,  82. 

Pythagoras,  theory  of  the  daily  motion  of  the  sky,  13  ;  his  teaching,  473. 

Quadrature  of  a  planet,  188. 

Queries  for  reviews  and  examinations,  475. 

Radiant,  325. 

Radius  Vector  defined,  181. 
Ranyard,  A.  C.,  his  theory  about  clusters,  347. 
Red  Spot  on  Jupiter,  240. 
Reference  Books,  list  of,  476. 

Reflecting  Telescope,  25  ;  explained,  41  ;  Newtonian,  41 ;  comparison  with  a  re- 
fractor, 42;  noted  ones,  43. 

Refracting  Telescope,  25,  34;  comparison  with  reflector,  42  ;  invention  of,  473. 
Refraction  by  a  prism,  28;  atmospheric,  117;  effects  in  total  lunar  eclipse,  172. 
Reticle,  136. 

Retrograde  Motion  of  a  planet,  190. 
Reviews,  queries  for,  475. 
Rice  Grains,  in  the  sun,  53. 
Right  Ascension  defined,  122. 
Ring  Nebula,  391,  444. 


INDEX.  337 

Rings  of  Saturn,  257-262. 

Roberts,  Isaac,  photographs  of  nebulae,  389,  391. 

Roemer  determines  the  velocity  of  light  in  1675,  252>  473- 

Rosse,  Lord,  his  great  reflector,  43. 

Rotation  of  the  sun,  58. 

Sagitta,  452. 

Sagittarius,  453. 

Satellites,  attending  upon  stars,  371  ;  of  the  planets,  472. 

Saturn,  distance  and  diameter,  253 ;  revolution  and  rotation,  254 ;  appearance, 
255,  256 ;  discovery  of  the  rings,  257 ;  divisions  and  dimensions  of  the  rings, 
258;  disappearance  of  the  rings,  259;  the  dark  ring,  260;  structure  of  the 
rings,  261 ;  stability  of  the  rings,  262  ;  the  satellites,  263 ;  physical  condi- 
tion, 264. 

Schaeberle,  J.  M.,  theory  of  the  corona,  81  ;  theory  of  the  markings  on  Mars, 
215;  observes  Jupiter's  moons,  249. 

Schiaparelli,  determines  rotation  time  of  Mercury,  196  ;  determines  rotation  time 
of  Venus,  203  ;  discovers  canals  of  Mars,  218. 

Schott,  Doctor,  39. 

Schwabe,  observations  of  the  sun,  59. 

Scintillation  of  the  stars,  337. 

Scorpio,  454. 

Screen,  used  in  solar  observations,  51. 

Sculptor,  455. 

Scutum,  456. 

Seasons,  in  middle  latitudes,  107;  at  the  equator,  108. 

Secchi  classifies  stellar  spectra,  353. 

Secondary  Circles  defined,  120. 

See,  T.  J.  J.,  investigates  the  origin  of  double  stars,  397. 

Serpens,  457. 

Sextans,  458. 

Sextant,  143. 

Shadows,  of  the  earth  and  moon,  170,  174;  shadow  of  the  moon  visible,  177. 

Ship,  the  position  of,  determined,  143. 

Shooting  Stars,  described,  308  ;  numbers,  322  ;  paths  and  velocity,  323;  masses 
and  constituents,  324;  radiant,  325  ;  the  August  shower,  326;  the  November 
Leonids,  327-329;  the  Bielids,  330;  orbits  of,  331. 

Shower,  Meteoric,  at  L'Aigle,  309  ;  the  August,  326  ;  of  the  November  Leonids, 
327-329;  of  the  Bielids,  330 ;  orbits  of,  331  ;  how  to  observe  a,  333. 

Sidereal  Time,  130,  131,  466. 

Sidereal  Year  defined,  1 13, 

Signs  of  the  zodiac,  100. 

Sirian  stars,  353,  354. 

Sirius,  its  distance,  349;  color,  352  ;  spectrum,  353  ;  double,  370,  417. 

Small  Circles  defined,  120. 

22 


INDEX. 

Solar  stars,  353,  354. 

Solstices  defined,  99. 

Spectra,  classes  of  stellar,  353  ;  of  the  nebulae,  388. 

Spectroscope,  description  of,  69-72. 

Spectrum,  the  solar,  74. 

Spectrum  Analysis,  the  laws  of,  73. 

Sphere,  the  celestial,  15-17;  position~of  points  on,  120. 

Spica,  369. 

Spiral  Nebula,  391  ;  real  form  of,  392;  location,  416. 

Square  of  Pegasus,  448. 

Stability,  of  the  planetary  system,  186;  of  Saturn's  rings,  262. 

Standard  Time,   134. 

Star  Finder,  465. 

Star  Light,  355. 

Star  of  Bethlehem,  201. 

Stars,  fixed,  defined,  i  ;  morning  and  evening,  191,  201  ;  number  visible,  336; 
scintillation,  337  ;  Milky  Way,  338  ;  tree-like  structures,  339 ;  constellations, 
340 ;  names,  341  ;  orders  of  brightness,  342 ;  magnitudes,  343 ;  catalogues, 
344 ;  charts,  345  ;  distribution,  346  ;  clusters,  347  ;  dimensions  and  nature, 
348  ;  distances,  349;  parallax,  350,  351  ;  colors,  352;  spectra,  353  ;  of  differ- 
ent spectral  types,  354 ;  light  and  heat,  355 ;  the  stellar  system,  356,  357; 
proper  motions,  358 ;  groups,  359;  motions  in  the  line  of  sight,  360;  the 
system  of,  363  ;  double  and  multiple,  364-372 ;  variable,  373-383  ;  nebulous, 
385 ;  hour  angle  at  any  instant,  467 ;  list  of  double,  469 ;  list  of  variable,  469 ; 
list  of  colored,  469 ;  proper  names  of,  470. 

Stationary  Point  of  a  planet's  path,  190. 

Structure,  of  the  Milky  Way,  338  ;  of  the  stellar  universe,  356,  357. 

Structures,  tree-like,  359^ 

Sun,  distance  and  diameter,  49;  how  to  view,  50,  51;  photosphere,  52;  rice- 
grains,  53  ;  faculae,  54  ;  general  appearance  of  a  spot,  55  ;  changes  in  appear- 
ance of  spots,  56 ;  dimensions  of  spots,  57 ;  rotation,  58  ;  periodicity  of  the 
spots,  59;  observation  by  Carrington  and  Hodgson,  60;  disturbance  on  July 
15,  1892,  61  ;  cyclonic  motion  of  spots,  62;  nature  of  spots,  63;  causes  of 
weather  changes,  64;  magnetic  storms,  65,  66  ;  the  storm  of  February,  1892, 
67 ;  frequency  of  magnetic  storms,  68 ;  the  solar  spectrum,  74  ;  constituents 
of,  75 ;  chromosphere,  76 ;  prominences,  77-79 ;  appearance  of  the  corona,  80 ; 
Schaeberle's  theory  of  the  corona,  81 :  nature  of  the  corona,  82  ;  light,  83  ;  heat, 
84;  causes  of  radiation,  85  ;  contraction  theory,  86;  past  and  future,  87  ;  con- 
stitution, 88;  the  midnight,  106;  enters  Aries,  112;  the  mean,  125;  cause 
of  eclipses,  173;  varieties  of  eclipses,  175;  partial,  annular,  and  total  eclipses, 
176-178;  duration  and  number  of  eclipses,  179;  its  path,  361  ;  testimony  to 
the  nebular  hypothesis,  402. 

Sun  Spots.     (See  under  Sun,  55-64  ) 

Superstitions  about  comets,  299. 

Synodic  Period,  of  the  moon,  146;  of  a  planet,  192. 


INDEX.  339 

System,  stability  of  the  planetary,  186;  of  the  stars,  363;  systems  of  planets 

attending  upon  stars,  371  ;  planetary,  401. 
Swift,  Lewis,  claims  discovery  of  intra- Mercurial  planets,  178  ;  his  bright  comet 

of  1892,  305. 
Swift,  the  satirist,  writes  of  the  moons  of  Mars,  222,  note. 

Tails  of  comets,  291  ;  evolution  of,  294 ;  types  of,  295  ;  of  Swift's  comet,  305 ;  of 

Brooks's  comet  shattered,  307. 
Taurus,  459. 

Telegraph,  used  in  determining  longitude,  142- 
Telescope,  management  of,  47  ;  reflecting,  25,  41,  42,  43  ;  refracting,  25,  34,  42  ; 

equatorial,  44-46 ;  Lick,  48 ;  invention  of,  473. 
Telescopic  Objects,  469. 
Telluric  Lines,  in  a  spectrum,  75. 
Tempel's  comet,  332. 
Temperature  at  the  moon,  166. 

Temporary  Stars,  denned,  374;  Tycho's,  375;  Nova  Aurigae,  381. 
Terminator  of  the  moon,  1 54. 
Theta  Orionis,  372 ;   condensed  from  a  nebula,  386 ;   spectrum,  398 ;    position 

of,  447. 

Thomson,  Sir  William,  on  the  future  of  the  sun,  87. 
Tides,  169. 
Time,  the  years,  113;    the  calendars,  114,  115;   mean  and  apparent  solar  days, 

125;    inequalities  of  apparent  solar  days,  126,  127  ;    sidereal  day,  128  ;    civil 

and  astronomical  day,  129;   mean  solar  and  sidereal  compared,  130;  relation 

between  sidereal  time   and  right  ascension,  131;    longitude  and  time,  132; 

where  the  date  changes,  133;  standard,  134;  determination  of,  by  a  meridian 

circle,  138;  sidereal  at  any  instant,  466. 
Tisserand,  investigates  comet  groups,  286. 
Titan,  263. 

Topics  for  Essays,  474. 
Total  Eclipses,  lunar,  172;  solar,  177,  178. 
Train  of  a  meteor,  313,  315. 

Transits,  of  Mercury,  197  ;  of  Venus,  204;  of  Jupiter's  satellites,  248. 
Triangulum,  460. 
Trifid  Nebula,  387,  391. 
Tropical  Year  defined,  113. 

Tschermak,  theory  of  the  origin  of  meteorites,  320. 
Twilight,  1 1 8. 
Twinkling  of  the  stars,  337. 
Tycho  Brahe,  his  star,  375. 

Ultra-Neptunian  planets,  277. 

Umbra,  of  a  sun  spot,  55 ;  of  a  shadow,  170,  174. 

Universe,  future  of  the  visible,  404. 


34°  INDEX. 

Uranus,  discovery,  265  ;  distance  and  diameter,  266  ;  revolution  and  rotation,  267 ; 
appearance,  268;  satellites,  269;  physical  condition,  270;  captures  the  Leo- 
nids, 329  ;  captures  Tempel's  comet,  332. 

Ursa  Major,  10,  406,  461. 

Ursa  Minor,  10,  406,  462. 

Variable  Stars,  definition,  number,  names,  373  ;  classes,  374  ;  temporary  stars, 
375;  Mira,  376;  Class  III.,  377;  Beta  Lyrae,  378;  Algol,  379;  Y  Cygni, 
380;  Nova  Aurigae,  381  ;  causes  of  variability,  382;  how  to  observe,  383; 
list  of,  469. 

Velocity,  of  light,  83  ;  of  meteors,  314  ;  of  shooting  stars,  323  ;  of  the  stars,  360. 

Venus,  morning  and  evening  star,  201 ;  distance  and  diameter,  202 ;  revolution 
and  rotation,  203  ;  transits,  204 ;  phases  and  maximum  brightness,  205  ;  tele- 
scopic appearance,  206;  atmosphere,  207;  physical  condition,  208;  transit 
first  observed  in  1639,  473. 

Vernal  Equinox  defined,  98. 

Vertical  Circles  defined,  121. 

Vespasian  jokes  about  a  comet,  299. 

Vesta,  discovered,  226;  diameter,  230. 

Virgo,  463. 

Visual  Angle  defined,  31. 

Volcanoes,  lunar,  157, 158;  terrestrial,  400. 

Voltaire  writes  of  the  moons  of  Mars,  222,  note. 

Vulpecula,  464. 

Watches,  135. 

Water  on  the  moon,  163, 164. 

Watson,  J.  C.,  claims  discovery  of  intra-Mercurial  planets,  178;  leaves  a  fund 

for  the  computation  of  the  orbits  of  asteroids,  228. 
Weather,  changes  due  to  sun  spots,  64 ;  effect  of  the  moon  on,  168. 
Wolf,  Max,  photographs  tree-like  structures  in  the  Milky  Way,  339. 
Wolf-Rayet  Stars,  353,  354,  399. 

Y  Cygni,  380. 

Years,  different  kinds  of,  113. 

Young,  C.  A.,  theory  of  sun  spots,  63;   observation  of  prominences,  79  ;  opinion 

about  the  collision  of  a  comet  with  the  sun,  300;  estimate  of  the  light  of  the 

stars,  355. 

Zenith  defined,  22. 

Zenith  Distance  defined,  121. 

Zeta  Cancri,  372. 

Zodiac  defined,  100. 

Zodiacal  Light,  334. 


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